To be clear, the head-scarf controversy in France seems to be less about separating church and state than about caving to the anti-immigrant sentiment taking hold in French politics.

As a legal matter, in America, even the most extreme view of the Establishment Clause wouldn't lead to this position. A public school does not "entangle itself with religion" by allowing students to wear identifiably religious paraphernalia to class. It would be another matter entirely, of course, if the school encouraged or required students to wear yarmulkes, Islamic head scarves, or JESUS SAVES T-shirts. That would be a clear violation.

Even if the Establishment Clause were hijacked and taken beyond its government-entanglement tether to support a wide-ranging eradication of religion from society, we also have the backstop of the First Amendment's Free Exercise Clause, slotted in to protect the right of individuals reasonably to practice their religion with undue government interference. The Establishment Clause forces the government to be secular, and the Free Exercise Clause forbids the government from forcing secularity on private citizens.

That said, don't think the headscarf controversy hasn't cropped up in the States. Take Oklahoma, for example, where a Muslim girl's hijab got caught up in her school's broad-application ban on headwear. Enforcement of the rule against the student is probably passable in this instance, notwithstanding free exercise principles, because the rule was not designed to target religious practices but to keep gang colors out of the schools. The Supreme Court held that the First Amendment does not entitle persons to win exceptions from "neutral rules of general applicability" on religious grounds. Thus Native Americans have to abide by laws restricting access to peyote, even though it might be crucial to tribal rituals or liturgies. And, of course (an easier case), Satanists can't excuse themselves from murder prosecutions by invoking a right/rite (pun there, somewhere) of human sacrifice.

So the Oklahoma rule is constitutional, though not terribly respectful of Muslims and Jews. Then again, schools are running scared enough of the Establishment Clause that they might reasonably fear that a lawsuit would follow from an express exception for hijabs and yarmulkes. It's the kind of suit that a disgruntled Christian activist group, smarting from school-prayer decisions, would love to bring.

Follow up points:

(1) Another headscarf controversy: Muslim woman in Florida refuses to expose her face to the Florida DMV for her driver's license photo. A tougher case here than a headscarf in school: covering your face would completely thwart the purposes of having a photo ID.

Some links on that:
http://www.courttv.com/trials/freeman/verdict_unveil_ctv.html
http://www.religionnewsblog.com/archives/00003432.html

(2) I'm bummed that my "Oklahoma" link doesn't work (at least not on my PowerBook, anyway). But it's http://www.cnn.com/2003/EDUCATION/10/11/scarf.reut/