When you are thrown into a cage with a tiger...yeah you can't run from it....
I hope Zhang Nan isn't too stupid to realize that he was in this situation because his family forced him into a school with a bunch of killers.
If he wasn't associating with a member of a rival clan, he wouldn't have been shot.
I bet that's why his mom left. She didn't want to be a part of the family pushing her son to embrace their dangerous heritage.