Suspend him for telling people they'll go to hell sure, but for the T Shirt? Poor form for the school and the Principal for continually asking him to stop wearing it. He's not affecting anyone's rights by wearing it, just the ass hurt feelings of some overly dramatic and ironically rights defeating atheists.
If the T Shirt read, "non believers you're going to hell" or something, maybe there'd be a better argument here. But damn, if someone wore a T Shirt that said, "a life is wasted without being aethist", I'd be like, yea bit of a weird thing to have on a t-shirt, but its your right and belief, cool. And I'm religious.