I can see what you’re saying in the sense that nothing should physically be stopping them from saying it, but also nothing should be insulating them from the consequences of what they say, right? To take it to a logical extreme, if a kid says they’re going to shoot up the school the next day, I hope we can agree that requires more from school admins than just “well, it’s his right to say that”.
I personally also think it’s stupid for a school to be involved for a shirt like in the OP (western society is much too puritanical about simple nudity/body parts, imo), but there’s clearly a line somewhere about what speech/expression can be allowed in public. Assuming you can agree with that, where would you want that line to be? I’d personally draw the line before it reaches threats to peoples’ physical/mental health (like the nazis and gore I mentioned).