It's assault, which is illegal, but not really sexual harassment. Report him to a teacher.

it is not sexual harrassment, because there no unwelcome sexual contact. its just assault and definitely tell a teacher or even the bus driver some people actually get band from using the school bus for certain behaviors.

its called boy being a**hole
i would have turned around and kicked him in the face

That I am sure would have made the situation so much better....

well probably because i had bad expiereinces like this girl was talking about
and the teachers and bus drivers didnt help me at all when i reported him so he kept doing it
i wish i would have had the guts to just handle it myself but i was only in 8th grade and i was pretty shy

ok thanks

and he has slapped my ass heaps of times be4 and i get 2 scared 2 tell my mum

Tell your mum. You are not to be treated like some object, which is what he's doing to you. He needs to be told to stop this before he does something worse to another girl. He's a toad.

Like Eiri said it is an assault, which is illegal, but not really sexual harassment. I would honestly report him to a teacher and if it gets any worse I would have a talk with the princable and see what he/she says.
No guy should do that! Guys like that are just jerks and just want attention.
Don't let him bring or get you down!