Buy the highschool level there is very little "mass attending" and when there is a mass it is fully accepted that the nonchristians being excused upon requesting so, or they can attend as non-participating parties.
The most religion you recieve is in religion class, however the religion class is typically more world religion.The exception is Freshmen year if confirmation is still being held then, but when I was in school it was a specific class and optional to attend if you were being confirmed.