Remember, what I said about rich-vs-poor families was only a generalization. I don't think you can say that a wealthy family will necessarily be more accepting, and a poor one less so. There are certainly very rich families that would still have a daughter who turns out to be a witch killed, and very poor, uneducated families that would still love their daughter too much to turn on her.
And what would a normal (normal on income, neither poor nor wealthy), (quite strongly) religious family do to an only son (no other siblings)?
Almost certainly. Especially with the state of the Muslim world today, it would take a very strong set of leaders to prevent such conflicts. If the students who have been there for a while are used to working with members of the other sect, and have gotten to know them, then tensions would probably ease, but they'd still have to keep dealing with new students coming in.
Do you believe that in a wizarding school, where the majority of the students are outcasts, the religious conflicts between Sunnis/Sunnites and Shi'ites would break out?