Could contact your local scenic construction company, lighting production company, party decorators and theaters to see if they have anything coming down that if scrap will otherwise be going into the trash. Given a 501c3 tax deduction status it might be worth putting it asside for you.
Otherwise, what about using, borrowing or making soft or hard flats for your maskings? Nothing says it has to be drape. If nothing else, does the school have any
portable walls or screens? Blackboards on casters?
If nothing else, what about some bed sheets? Given they are flame resistant, perhaps you can get some as donation or at a large discount than dye them black.
Just remember that no matter your budget, you still have to comply with fire
safety codes and this would include not using plastic tarps and similar things for
drapes unless fire resistant. You also might have to do a flame test.