I used to get picked on all the way through school, I mean up until I left school at 16 it was just constant name calling or stealing my things or whatever.
What you need to remember is that those kids have major insecurities which means they feel the need to vent this onto people who appear to be weaker or vulnerable and who they can offload their crap onto. Bullying makes some people feel more secure with themselves and their status position if they are the bully rather than being the bullied. Wouldn't you feel more secure too? It's like defending a castle or something or emotions. They don't want people to see the real person inside, so they cover it up with name-calling of people who are more vulnerable. These types of people are weaker than you are.
You need to remember that they are not better than you, nothing makes them better than you. Not money, not designer clothes, not anything. Everybody is born and dies an equal and there is nothing which dictates that they have more power over you. You need to hold strong. "Sticks and stones can break my bones but names will never hurt me." can only hold for so long, I know, but you do need to remember that those kids are going to get a harsh reality check when they leave school and they behave like that in the workplace and get fired as a result of it. They won't survive two seconds outside of high school, you will!!!
If you are being bullied, it is always best to tell somebody. I know that sounds harder than it is, but really it does sort things out. If somebody bullies you because you told, then report them again. If the school doesn't intervene then get the police involved. I have taken bullying cases right up to the principle and to the point where the police were going to be called in before the bullies finally relented and chose somebody else for their target.
Another thing to do is to NOT SHOW IT BOTHERS YOU. The more they see that stealing your books bothers you, the more they will do it. They wouldn't dare actually throw away your books because they know that would go further than "a bit of harmless fun" and you will find your books again, even if they are in the hedges. The more you show they don't bother you, the less they will do it because they will grow bored of seeing no reaction from you.