Here's a comment I posted over at TalkLeft on the report of the Justice Department seeking to defend the right of a Muslim girl to wear a headscarf despite a district ban on head coverings.
Anyway, it seems to me that the key here is that the school's policy is overly broad in pursuing its stated public interest; namely, the suppression of gangs. The Supreme Court has already held that if there is a bona fide public interest in banning a particular practice, it doesn't matter whether some people happen to use it as a religious practice. The test case here involved banning peyote as a drug even though a particular Native American nation used it in religious ceremonies. But if the regulation is too broad to be justified given the stated interest, or so narrow that it obviously targets a particular group or religious practice (imagine a ban on eating unleavened bread wafers on Sunday mornings for health reasons), then it is unconstitutional.
In this case, a ban to prohibit all or any religiously mandated or inspired dress because it was religious per se would be unconstitutional. On the other hand, if, in a community, headscarves were judged to be a symbol of insubordination against a teacher's authority (much as some gang symbols are) then the school could take action to ban them, even if well-meaning Muslim girls were "caught in the crossfire." On the other other hand, simply being a member of a religion cannot be interpreted as sign of insubordination, even if the religion's ideology itself is explicitly rebellious or defiant of secular authority, as in the Nation of Islam or certain fundamentalist Christian groups.
As you can see, as you start to split hairs in these questions, you start relying more and more on notions of "good faith" and "reasonableness." My own tendency is to want laws that don't depend on these things, because you want them to continue to function even in the worst-case scenario, but it's also very instructive to see just how important it is for our government and our citizens to act in good faith and not try to break the system.
Probably, if you tried really really hard, you could come up with a way to constitutionally ban headscarves, just as for a hundred years people found a way to constitutionally discriminate against African-Americans. But you're breaking the system when you do that. It's not so much a legal question at that point, because how can the law be illegal? More of a cultural and social question. This question: are you willing to support the rule of law and equal rights, or are you really trying to aim for their overthrow?
--Melissa O, at 15:33