Sorry but as a step parent you have no rights. Is your fiance really that afraid of her reaction that he won't take you to a parent teacher night? At some point BM has to grow up and accept that you are in her child's life half of the time. 

My SDs BM refuses to meet or acknowledge me as well. We have SD a lot less often than you have your SS so it is less of a necessity to meet me. However it's been almost 7 years and she still pretends like I don't even exist. I'm not banned from SD's events but if I'm in the same place as BM she will look like right through me or completely ignore me. One time we are standing in a parking lot exchanging SD and my DH introduced himself to SD's stepdad but being turned heel and left without looking me in the eye. The whole time she had her body turned as if I was invisible and not even standing there was part of the group. 

I think that after a while of acting this way it becomes too awkward to change it. Even if the status quo is uncomfortable it is more uncomfortable to try to change it.

Why would you be interacting with the teacher and not his father? 

How funny that BM and your fiance are okay with you doing the transportation, homework help, etc. but you cannot show your face at a parent teacher conference.  I would politely tell fiance that if you aren't welcome to meet the teacher at back to school night you will need to decline helping in any aspect with schooling.  You're being treated as the paid help (chauffeur/tutor) without the pay.  

Who says you're "not allowed?" You are not a child and you don't need permission, especially permission from BM for anything.

If you and your DH have agreed that you have significant responsibility for this child (and I hope you did agree, and his care wasn't dumped on or simply expected of you), then you need to be part of the educational support system, which includes school information sessions, etc.

DH doesn't get to hide you AND expect you to care for his son. This is a DH problem. And if BM makes a scene? That's on HER. It's not his responsibility to control her either, espcially at your expense. 

I wouldn't do alll this for skid. No rights, no responsibilities. His real mom should be doing all the school stuff! You'll regret throwing yourself into this role, especially when you have your own child, trust me.

If you can't go to back to school night.  Then tell DH. He's handling drop off and picks up. And homework. He wants it his way he has it 
Get a Job. Not working from home. Save your money 

DH and BM don't get to pick and choose when you can be involved and not be. If they are both dead set on you not going to Meet the Teacher Night, then you need to decline ALL school matters. Transportation, homework, communication with emails, packing lunches etc. Do not do any of it. 

IMHO, BM still hasn't accepted you because she is insecure. Especially if you are taking on a parenting role, she probably feels inferior. You're obviously not a temporary girlfriend (I could see not wanting to meet then). BM just has a problem facing reality. 

Just a question, has your bf always had 50/50 or is this a recent thing?

Lets discuss a tiny fact.

1. UNLESS there is a protective order against you, you are free to go where you want to.

Besides that I would let bio dad and bio mom raise their child. THEY do homework, THEY arrange school pick up/drops. THEY do the Childs laundry. THEY go to medical appointments.  Guess what THEY talk to teachers. NOT you ...I am shocked you are allowed to without both mom and dads consent.

Ma'am it is their moral and legal obligation. Why wont you let them do what they are required to do?

Your problem is your SO, not BM. It sounds like he is the one telling you that you cannot go to the school function because you might see BM and she might cause a scene. That is on him. He has alternatives that he is choosing not to take. He could coordinate with BM by telling her what time you and he will be there - then it is on her to decide if she wants to see you or not.

You also have a choice to make. If your SO is not willing for you to attend the school function, then you should refuse to do anything pertaining to school. Has he looked into the school policies? Many will not release a child to someone unless they are on an approved list - has he taken care of all that documentation?

Sometimes Indifference is better than hostile engagement.  If you never meet her.. fine.  Your husband can  pass on any relevant information regarding the school to you.. etc...

I might ask him how he will feel when his child graduates.. gets married.. will you still be asked to stay behind though.  I mean, what is happening now is not anything you need to necessarily be part of.. but those events?

Welcome to being a stepparent...

You make all of the compromises with none of the reward 

Are you sure you want to open Pandora's box? Your SO has hinted at BMs reaction. He may be minimizing what she is really like so to not scare you off. Your SO may be trying to protect you from ever having to deal with or interact with BM especially if she is an HCBM. 

This is just a thought but ... 

" I bought a new house within spitting distance of hers "

Have you considered that she might feel that your move to be near her was something that she maybe didn't welcome? That perhaps she felt you were trying to "rub her nose in it",that she might consider your move to be "pushy" that she might consider that you're flaunting your happiness? If this were the case, it might seem obvious that she would rather keep her distance, wouldn't it?

May have already been said I just scrolled to the bottom. 

Complete 2 meet the teacher conferences one for her and one for you and him. THAT simple. 

If she doesn't want to meet you fine. Then she gets the full brunt of excluding herself from how the operation works in your guys home. 

And if Daddio cannot agree to that than maybe you should find your happy travels back where you came from and continue your life in a more familiar, appreciated, well liked place. 

... is just plain terrible. You're being treated as the nanny and the bedwarmer, and you're pregnant to boot.

The kid has two parents, and you're not one of them. In your position, I would do absolutely nothing for this kid. My pregnancy would suddenly become unbearably uncomfortable such that I could not raise a finger except to take care of myself. I would be indisposed, constantly in bed with my door shut, watching soaps or reading trashy novels. I might develop a case of pregnancy-induced wanderlust and need to be out of the house by myself for many hours a day.

To avoid the wrath of CPS, which will not understand why you ignore and neglect a child in your care, do not allow your husband to leave you at home with his kid while he runs off and has fun with his coworkers. That's what child care centers are for, and you're not a child care center. Leave the house before he goes to work if necessary. Be friendly and sympathetic to your husband's spawn, but politely decline all offers to do anything whatsoever for the little darling. Take only the mildest, most distracted interest in his school, medical and extracurricular activities. Start this program yesterday.

When your husband realizes that no, you are not involved and HE is responsible for seeing to the welfare of his kid, he will be madder than the dickens and will complain bitterly. But it'll start getting him used to your being completely unavailable to pinch hit when your baby demands all your attention, which I hope you will give him. Because your DH has shown himself to be a poor excuse for a parent by trying to palm his duties off on someone not related to his kid and who has no skin (or DNA) in the game. I hope he treats your baby better than that.

I'd wish you luck, but what you're really going to need here is a strong backbone.

While Meet the Teacher night may seem like a huge deal, it's not. Count yourself lucky that you get to stay out of that crowd. Fiance needs to get his own copies of any handouts to bring home. All the relevant info will be printed for home reference.

Before that night, Fiance needs to call the school. (Most schools have office staff the week before the first day.) He needs to find out exactly what he needs to fill out so that you can pick up on his custody days. If he wants the school to be able to talk to you on his days, is there a form or does he need to write a letter? Does he need to show them the relevant part of the custody order? When can he do this? This can vary by district so what one district wants may be different from another. 

That night DH needs to be sure that he is signed up for the school parent portal. He needs to talk to the teacher and tell her he'd like his own copies of notices sent home with children. 

DH needs to set this up and navigate BM's response. It's his responsibility to see that you are on the pickup list. If he doesn't, they won't let the child home with you. Here, that can't be called in. It has to be in writing in advance. Even though it's his day, they'd call BM before they let you take the child home if you are not already on the list.

Meet the Teacher night is not where I'd put my foot down. I'd stay out of it and make Fiance do the work to set things up correctly himself. My hill would be the holiday concerts and performances. 

You'll be picking up 50% of the time. You'll get to meet the teacher multiple times. Don't stress about a one night cattle call event.

As a Sparent, you have no rights, except every right you choose to take. And no one can stop you from taking those rights. Officially you have no rights, but neither can the blended family opposition prevent you from taking any and every right you choose. Practically, neither can the courts.  What Judge would take exception to a SParent being an active positive participant in a Skid's life? What the X thinks, wants feels, etc... does not matter. PERIOD! DOT!

The right to be at the side of your equity life partner anywhere and everywhere all of the time is yours and yours alone. So, go to the teacher event.  If your DF tells you no, find a man with balls and who knows how to keep his X failed family breeding partner in her place and how to recognize and honor his equity life partner.

His X has no rights to any place in  his life. That failed family .... well.... failed and no longer exists.

This is a hill I would be willing to die on with zero tolerance for my mate to not defend our family and marriage from his toxic X any time they crawl out from under the slime covered rock they occupy at the bottom of their shallow and polluted gene pool.

Together... you and your DF can protect and defend your marriage, family, and your SKid from the toxic X/BM.  I would be at each and every PT conference, every seasonal event, and I would make rubbing the X's nose in my presence a given that the X cannot avoid.  If the X freaks out, I would be rubbing their nose in their own behavioral melt down.

Once my DW and I married I was at every kid event, every PT conference, every court hearing, every lawyer consult, and nearly every face to face SKid hand over.  The SpermClan knew that if they fucked with my wife or my SKid that they were fucking with me and that every bit of my intellect, assets, and access to top notch legal counsel would smack them in the face.  They learned to stay under their slime covered rock at the bottom of their shallow and polluted gene pool.  For the most part anyway.

Go, be at your DF's side.  Make sure the teachers know you from day one.

I would. In fact... I did

My SKid thrived with that support and has left the shallow and polluted half of his gene pool far, far behind him as an adult. He is living an outstanding life.  His half sibs by two other baby mamas are living miserable lives at best and are in prison at worst.

Be your DF's equity life partner and tolerate nothing less than being his.

Good luck.

As for meeting your DF's X, why would you give a shit about her at all much less feel some deep need to meet her and for her approval and recognition of you?  Stop that.

Im 7 years in this and last semester, SD15 backstabbers freshman year of high school, I did what you dio/did. Drop offs pick ups and got heavily involved with schooling.

Then came the accusations and lies and manipulations. So - you have a partner problem. You need to re-read the advice given here and formulate a plan of action.

good luck!