Criminals in class

I go to a school in Canada and I have two criminals in my class, One sits right behind me in LA he was arrested

1 for Assault

2 for Possession of illegal substances

3 for Driving a motorized non insured vehicle on a public road without a license

he picks on me eg. Shoves me (AKA Assault), Likes to steal my things in class or "borrow" in his words. I do not like this some days he is a real jerk and others he is kind of nice... In the extent of the fact that 1 and 2 define him.
     What I am trying to say is that he is not nice even when he is his nicest.
I want to know what I should do when he shoves me. I have tried telling the teacher but he got a 15 minute DT at lunch later that day at the bus stop he pushed me by the three steps, luckily my friend caught me before I hit the ground. I didn't tell the teacher and the next day nothing happened. Besides the fact that he gave me a high five when I pretended that I stole a cupcake from my brother.

I want to know you opinion or story.
What do you think is going on in his head and should he sit one desk behind me?