If it gets in the way of students learning in an appropriate environment. (That’s why students aren’t allowed to talk out of turn in most classrooms.)
It violates the rights of others.
If it can be categorized as slanderous or libelous speech or bullying or harassment.
It encourages illegal behavior.
An example would be a t-shirt that promotes drug use.
It’s vulgar or obscene.
This is a tough one because what a school principal thinks is vulgar or obscene may not be what a high school junior thinks is obscene. Just because it’s about sex or homosexuality, for example, it’s not necessarily obscene or vulgar.
It’s part of a school-sponsored publication, brochure, web site, or other type of media.