Why do Muslims say “Peace be upon him (PBUH)” after saying the name of a prophet of Islam?

I don't think it is, neither is it legal, think about how the prophet Muhammad (pbuh) would deal with it, I don't think he'd go on a killing spree killing all the people who disagree with him. Try talking some sense into your classmates, tell them about Islam and correct they're insults. If all else fails, I suggest you just ignore them.

Not kill them please?

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.