In all honesty, I think that the schools actions are justifiable, up to a certain point of course. If there is a really frum, yeshivish school and a child applies who is not so frum and yeshivish and him being in the class would probably expose these really yeshivish kids to things their parents don’t want them being exposed to, the school should accept that child? (sorry for that run-on sentence)
I think the child should try to find a school that has kids more his type.