The Constitution is the supreme law, but it is not the only law. There is, for example, no prohibition against murder in the Constitution.
Does any government have the right to tell you where to go, when you must return home, when to go to bed, what to wear, what to read, what to watch on television? Parents are not only allowed to exercise that kind of control over children, but they're irresponsible if they don't. Schools take on some of the duties of parents when parents entrust children to their care.
The Constitution doesn’t apply to teens or children. Learned that in hs history. And the laws never applied to teens, children so there’s that.
Anyway, they don’t apply since children aren’t held responsible or accountable in most cases so the rights given in our constitution don’t apply to them.
Children do have to adhere to the code of dress/conduct in schools, even public schools. But their parents can fight it and have fought it. This kid’s parents could fight the principal’s making him take off or not wear the cross. And if they wanted, they could easily win.