How about a t-shirt with a funny slogan like
"I see dumb people" (from thinkgeek.com)
Once you post something on the web you cant get it back, think twice before you do anything. Those kind of get back at the bully plots never work, why not go in to the office with the gym teacher and do some schedule work arounds so you dont have to see each other, or sit at opposite ends at the room. Once the school knows about what is going on all will be solved, just be careful, who ever goes to the office first is generally the one to be "believed"