Well, grades are important.
It doesn't matter if those girls were offering sex or Friendship, it only matters how Norman "interprets" what they said/did, and what he feels is a justifiable punishment for the gap between what really happened and what he thought happened.
Norman looks like a school yard shooter. Even if it's subconscious, or just Darwinism in action, it's a relatively inexpensive defence offensive called "being nice and inclusive" which limits the number of whackjobs unloading their dads ak47 into the lunch-room because there's no one to revenge upon.
The future is now.
Or is it possible that their parents raised them right in a small time with no big city influences and they are nice kids like it was still the 1950s all over again? Like a less extreme version of the Village.