What to do if people are always trying to mess with you?

Im in 10th grade right now, and people are always trying to pick fights with me and stuff. Now im a pretty big dude and people are always saying **** like "fatty" and what not. I know its true, but it still pisses me off. The other day my wallet fell out of my pocket and someone took it. I dont know who did it but it just makes me want to beat there face in. My point is people are always trying to mess with me and ask me to fight, i dont really want to but they just call me "" and "loser." Sometimes i feel like i should just fight to defend my reputation. but i dont want to get expelled, and school staff doesnt do **** about it. (Even if i went and told, i would be a little ***** and would rather get harrased then be a tattle tale) im lost, does anyone have any ideas?