To accomodate every religion, schools would have to bar activities on Fridays, Saturdays, and Sundays. Of course, they should also avoid Wednesdays because many Baptist and Chirch of Christ churches have services on Wednesday, as well as Sunday. And many other Christian churches hold choir practice on Thursday nights, so that night is out.
My reckoning shows that we have only Monday and Tuesday left to hold sports events. Of course the secularists will object to school games and contests on Mondays because that would interfere with Monday Night football -- another type of religion.
So, it looks like everything will have to be planned for Tuesday night which will surely prevent a student from participating in more than one activity! ;^