This dawned upon me while I was in school and I saw a friend of mine wearing a "I <3 Jesus" shirt. So I started thinking "So wearing a christian shirt to school is fine, but I wore a shirt that said 'Im Athiest' or 'Im Agnostic' I'd have every one down my throat telling me to change my shirt or go home" why do you think this is?
I think there's a difference here. School is a place meant to work without disturbances [though it rarely ever does] -- For this reason, I don't think it would be proper to wear a shirt that is insulting like 'Omg I killed Jesus' -- That's messed up. I'm Muslim, but I believe anyone can wear their own shirt to school. I've worn my Muslim Student Association shirt multiple times and it has a few pieces of the Quran on it, and nobody has ever asked me to take it off. I've heard a few terrorist jokes or questions of curiosity from friends, but that's all
I think wearing a no-religion shirt is okay as long as it isn't mean -- except, I suppose that is kind of hard for an atheist considering I don't think there is much to put on there. Since you can't write 'There is no god, get over it!' or 'he's been gone for 2000 years, HE'S NOT COMING BACK', etc.
Also, if a student wanted to, he could probably fight for the right to do so -- it's just that, nobody would go through the trouble.
