Once in Elementary School, I stabbed a kid with a pencil because I thought he was making fun of me. I barely received any punishment for this.

Around the same time at the same school, my mother was called into school on account of my brother's bad behavior. Apparently, he was acting out by dropping crayons on the floor. That was his main crime.

I do not understand how schools determine how serious behaviors are.

Simple. If the behaviour bothers the teachers, then it's serious. Kid gets stabbed by a pencil? Eh, so what? Boy drops crayons and being annoying? Detention.