The real test is wether they would allow a Mexican, Arab, or African flag on a purse. If they would, then they are hypocrits, and violators of the first amendment. If all such symbols are banned, then they at least aren't hypocrits.
The legal test is whether the "speech" is substantially likely to cause a significant disruption to the school's primary purpose...
(which is education, in case they've forgotten...)
Most of those flags are not likely to cause a disruption. However, since liberals have pioneered the method of screaming at the slightest violation of their tender sensibilites, they create their own Free Speech rules, whereby they can restrict your "speech" in school simply by reacting disruptively towards it.