Irena,
It sounds like you need to think about putting this in place through your ds' 504 plan or IEP, rather than just suggesting or buying apps for the class to use. Thinking beyond just apps, what was really important for our ds (even in elementary school) were four key things:
1) being able to have access to his AT across subjects, not just when a cart of tablets was in the classroom available to the full class.
2) being expected to use his AT rather than having it be optional - this included having it written into his IEP accommodations. The accommodations were never software specific, but stated that he could use word processing for all subjects and for any written assignment longer than 2-3 sentences.
3) being able to take the AT device (used to be a laptop, now an iPad) back and forth from school to home.
4) having access to his specialized AT software (apps) over school holidays and summer vacation. This was important when he was younger because he was learning how to use typing, word-prediction etc and we practiced over the summer - not tiger-mom practice, but easy-going getting him used to using AT practice. Now that he's older, he needs to use it every now and then over the holidays and summer. Realistically, his AT is always going to be a part of his life, so taking it out of the loop of just being a "school" device helped ds get used to the idea of relying on it.
Note - reason "number 4" isn't quite as key as the other 3, but it helped us make the decision to purchase the device and software/apps ourselves, rather than rely on ds' school to provide them. It was also helpful to us to own the device and be able to load whatever we wanted to on it so that we could try new software etc when we felt it was needed, rather than go through lengthy negotiations with school staff followed by lengthy AT evals, each of which had to be done one at a time. But I digress! Sorry about that

Re the math - Panther Math Paper is the best app I've seen for what you're looking for - and it's $20. I am not sure how app purchases for schools work - are they able to make one $20 purchase and share it with all the school iPads?
polarbear