My schools not giving exam leave either, which isn't any good for me because I can't revise well with people around me or when I'm in a class environment. However I can see why they're doing it, because those students who are not determined in school and won't bother revising at home would be better going to school where they are placed in an environment where learning is compulsory. It might be worth your parents speaking to the school about you staying home to revise, if you feel that it would be better for your own education, then i don't think the school should have a problem with it, if they know you're a hard working individual.