Gosh, I've gotten my cards taken away by teachers on so many occasions...be careful with that! As far as what tricks to perform: I'd recommend learning magic with ungimmicked decks--regular cards. For a couple reasons:
1) You already have plenty of school stuff to carry around in your backpack.
2) The other kids won't be as respectful to your props as an older, paying audience.
3) With a regular deck of cards, you can perform lots of different tricks, as opposed to most gimmicked decks, which only allow you to perform one trick.
As far as which tricks to learn: Short ones are good starting points. Like the "Ambitious Card." You'll want to stay short because you might not have time to, say, perform a 7-minute poker routine before the bell rings and everybody scurries off to class. But a trick with many small parts (like Ambitious Card) can be stopped at any point, and you'll still pack some magic moments in there.
Good luck!