adults have the right to free speech, but welcome to real life. my daughter works in a bank. if you wear a shirt with any words on it, you're fired. it doesn't matter what the words are. they're not saying you're not free to own it or wear it on your own time - they deem such attire inappropriate IN THEIR BANK.
the school didn't take the kid's shirt and burn it. and a shirt isn't speech. our local schools here have dress codes. religious references are generally out, among other things. and you DON'T have freedom of speech in school - try a little freedom of speech out on any teacher and find yourself suspended. you can't say anything sexual. you can't say anything profane.
there are MANY things you cannot wear or say in most schools - and most christians applaud most of these restrictions.
ya can't have it both ways, christians. you don't want dirk the skater wearing a naked chick on his shirt? or melissa the cheerleader wearing a shirt that says "fuck christmas"? then you don't get to wear a shirt that is trying to shame other people, and that forces your religious beliefs onto people who cannot leave the school and are forced to see your shirt.
aside from my personal disgust at this kid with the shirt, you can't have a dress code that restricts everything except what YOU want.