There's a few ways. If it's a laptop get a wireless router and pick up a signal away from your school. Don't share any of your files, it's so easy to pick up on a connection constantly pinging multiple places, so if you aren't sharing it's harder for them to spot. Set yourself up with a dynamic IP. The problem is, I don't know how they're picking up on you, so it's harder to tell you how not to be seen.