You've come to the right place.  We all rant and rave but we all pick up words of wisdom, too.  It sounds like you're seeing your situation clearly.  Hoping for your best possible outcome.

I hope you take what I say to heart.  You should never care about your SS more than the bio-parents because it will only lead you to resentment them and cause you emotional pain.  If they want their son to do well in school, THEY need make that transpire.  If SS needs to be picked up, THEY need to make that happen.  You see what I'm getting at here?  Plus, stepkids want their parents to be involved in their lives.  Not the stepparent.

I've got almost 20 years into stephell.  I learned the hard way that I needed to step back and step away and let the bio parents figure it out.  I was done being taken advantage of.

Good luck and welcome to steptalk:-)

Fellow SM teacher here! I feel your pain. Just because I'm a teacher, all of the school responsibilities fell on me when DH and I first got together. Even wanting me to email SDs teachers, complete paperwork, do homework, etc! It was ridiculous. It all changed during COVID. I had my OWN classes to teach online, let alone do SDs schooling as well. I also became pregnant here too and had my own little one to worry about. It was finally time for DH and BM to sink or swim. They usually do more sinking now than anything else, but oh well. DH has even said he "doesn't see SD13 making it to college." I definitely agree, however it's no longer my problem to try to "fix". 

No one else will look after your BS like you will, so make him your priority 100%! Also don't hesitate to say it. 

You are not the live in tutor for their failed family progeny.  So stop being their rescue ranger when it comes to their parental failures. 

Set the standards of behavior  and standard of performance for your home for the kids and for your DH.  Enforce those standards.

You have a toddler age special needs child.  SS is not your parenting priority. Not your parenting responsibility at all. In fact, your DS3 is also your DH's BK.  Daddy needs to extricate his head from his own ass and his duty of kid priority is his special needs son and more importantly his daily priority has to be unequivocally his wife and his marriage which includes being an equity life partner and an equity parent to HIS special needs son.

Take care of you, take care of your own child and make sure your DH has clarity that he and BM are the ones who are on the hook for parenting their child.

Since DH has already extablished and is proving that he is missing his responsibilities as your partner and your special needs child's daddy.

DH and MIL need clarity session that your child has every right to daddy's and G-Ma's love, concern and focus.  A 9yo has to behave as a 9yo. Adults owe far more to a special needs 3yo than to an attention seeking coddled 9yo.

IMHO of course.

DH also owes you support and companionship as you provide the primary parenting to your shared DS-3 and work on completing your undergrad degree.

Though I am the SP in our marriage, DW was an undergrad student when we met and when we married. SS-31 was 15mos old when we met. We married the week before he turned 2yo.  I completed my undergrad 5mos before we married.  DW completed her dual major undergrad 5yrs after we married, I completed my MBA the next year, she completed her MBA three years after that. We partnered in life, careers, raising SS, and in our education endeavors. 

Do not accept less from your DH. In fact, expect more. He is the father of a special needs toddler.  Your child deserves a father who delivers and not one who whines with "I don't know what to do." when his special needs child needs his care.

Set and enforce the standards of behavior and standards of performance that you hold the adults to and the kids to. In an age and capability appropriate manner of course.  Keep in mind that the HCGUBM is irrelevant. Your DH owes you, his eldest child, and your toddler to keep his X firmly in her place.

Good luck.

Welcome by the way. I hope that you find this to be a good place to vent, contribute, and to pick up some useful advice from others who are living or have lived the bended family SParenting dream.

Take care of you.

DH and HCBM are lazy parents and expect me to pick up the slack without any of the rewards or benefits.
 

One of the major reasons I won't be dealing with men with dependents in the future UNLESS they are wise and make dealing with them/their ex breeder/ and their spawns worth my while. I'm not doing step life for less than $100k / year. 
 

Call it ruthless or whatnot but I value my peace and my wallet. 

Party with BM. ?   If so that the first mistake.  DH divorce BM. I n doing so, and starting a relationship with you, He does not get to play happy family with SS and BM.  If he wants a Birthday party for SS. Then he should do it with OUT BM.   I would never put myself is the situation where I am the third wheel. Except for SS Graduation and Wedding.  
'Your DH must understand he wanted his DS to live with him.  He has to do the lifting. He makes the cake. If he had enough time to buy the cake mix he should of made the time to bake the cake.  He should be helping SS with his homework not copping out to you.'

You should make a list of DH duties for DS.   What he HAS to do every day. Feed him dinner, bath time,     So he can't not stop do it in a  week or two. I.E. can't get out of feeding him dinner . Giving him a bath. Putting him to bed.  You have a problem with DH. You must put your foot down, Make him pull his weight. Stop kissing BM as*   
'Have control of your home and life