Until the bus pulls away the driver can write you up. If the driver would see you making obscene gestures or hear comments as they drive away they can write you up. And your parents should punish you for being written up. So start to act like a grown up
You are considered to be on school property until you arrive home. If you were to get off the bus, the bus drives away and you beat up a kid before you go home, you are still considered "on school property" and the school can punish you as they see fit for fighting, even though you weren't at school or on the bus.