actually, the ACLU has been involved in several cases of this ilk and HAS already commented on this case.
the case law (and i am paraphrasing), which the ACLU has commented on is basically this...
for a school to ban speech like this (and certainly - wearing a flag shirt is content speech), the burden is on THEM to show that the danger of allowing same speech is both imminent and clear. again, i am paraphrasing - i haven't read the case in about 2 yrs, but that is the gist.
so, actually you are wrong. the ACLU ***has*** stood up for the rights of the kids to wear the flag shirts. already.
now, what would be problematic would be if they banned mexican flag shirts and allowed US flag shirts, or vice versa. THAT would be a MUCH MUCH (probably impossible) burden for the school to justify for a # of reasons...
now, the constitutional laws are interesting, but the commonsense rules also apply
for any school to ban the wearing of a US flag shirt based on potential fears of dischord/retaliation/violence shows the larger problem is that there may actually be a high antiamerican sentiment among many if the mere wearing of a US FLAG shirt **in*** the US leads to violence. that's pretty frigging insane
these are american schools. the flag is all of our symbol. every one of us (well, at least EVERY LEGAL CITIZEN)
far more disturbing for me as a free speech advocate is the move by many stalinist left wing school boards (seattle comes to mind) in attempting to ban military recruiters from college and high school campuses, for a host of reasons - usually the claim of the "homophobic" policies and/or a general antiwar antimilitaristic justification
regardless of what one thinks about the military (and clearly under the solomon amendment, not mention the 1st amendment - this is insane and stalinist and unconstitutional to try to prohibit access of the military recruiters to our PUBLIC campuses), they have the rights to express THEIR points of view and recruit. the proper remedy for those that disagree with the military in general, their stance towards gays, or their choice of berets is to PROTEST and counter the military speech with THEIR speech, not to ban the military. in a FREE society, we counter speech with speech. we don;t ban speech. these stalinist campus thought police don't understand that.
it is also (no pun intended )

very paternalistic ESPECIALLY in colleges (where the vast majority of students have reached the age of majority) to claim, as many do, that children/students need to be PROTECTED from the military recruiters.
so, apparently free choice (like to choose to listen to recruiters and choose to join the military of one's own free will) is a sacrosanct right in regards to a 16 yr old girl aborting a fetus without even parental notification, but is not even a consideration when allowing that same 16, 17, 18, 19, 20 21 etc. year old girl to hear a recruiter from the US mIlitary and decide for herself whether she wants to choose to serve her country.
the irony is so thick i could cut it with a ladle
now, on a PRIVATE campus, this is a non-issue. if a quaker school, for instance, wants to ban military recruiters (probably due to their pacifist stances), this is entirely constitutional, since they are PRIVATE schools. now, if they receive public monies... welll.. i digress
