As a BM myself, when my son was in school, I never expected anything - as that is what child support is for. The only expenses I ever asked for help with were the big ticket items like braces...
As a SM - it is quite the contrary. My SO not only pays significant child support, he is still expected to pay for all supplies, clothing etc. His payments went up this year, and BM found a way to fund a two week trip to Disney World ... but neither SD has clothes for school which will start when they return. I have made it clear that this will not be an expense my SO better pay. Then again, it's hard to allow the child to suffer for the BMs lack of money management.
JMO...

You didn't mention if your SO pays CS. If he's got a 50/50 parenting time and no CS 'fair' would be actual school supplies every other year (her turn this year) and each home purchase what their household needs as far as shoes/clothes. Something like uniforms for school might be considered 'extra' on top of everyday clothes and each home might be each buy one seasonal each , meaning two each (spring/summer and fall/winter)

I your SO parenting time is split differently and SO pays a decent amount of CS plus he bought last year, I'd think it was her place to worry about it. Help if actually needed, but not just because BM doesn't want to pay herself. As far as the yearbook, I consider that an extra and 50/50 each isn't out of line to consider doing.

Your problem will come if BM refuses and SO refuses. Which one plays hardball and which one caves. The child can't go without. Or if BM buys some and SO wants to also purchase some on top of it as an extra, unless it's your personal money or household joint money, you've really got to keep silent and let him. How he spends his personal money on any extras is really up to him. Though that idea does cause a lot of friction in some households. The important bottomline should be that you, yourself, aren't purchasing and supplying...that's the parent's responsibility.