Whatever you do to make decisions will leave someone unhappy. I believe that in the UK, an officially Anglican country, a Scapular Medal would be permitted despite a "no jewelry" restriction. At a private Baptist academy in Texas, it probably wouldn't be excepted. Of course, private institutions have much greater latitude in these things, so the question is about state-run schools.
Personally, I like the compromise we have in our US Public Schools. If a girl wants to follow the "Silver Ring Thing" practice, that's her decision and the school can't stop her. If there is policy against jewelry, a letter from your religious leader, or even a parent is often enough to get an exemption.
The downside of this is that you can be too permissive. After someone says, "I'm a neo-pagan and nudity is part of my religion!" you draw the line. Then, maybe you're called "unfair." Sometime you are being unfair. That's why groups like the Christian Legal Society (and the ACLU) exist. CLS has defended many, many Christians whose legitimate right to religious expression has been infringed in the public square. The ACLU has even defended a handful of Christians, as well as people from other faiths.
Things work differently in other countries. In the UK, as I've pointed out, you don't have the right to wear a silver ring and call it "central to Christianity" if the Church of England disagrees. In Turkey, the post-Ottoman government is so militantly secular that Muslim women are forbidden to wear hijab in government buildings — this is in a predominantly Muslim country. I don't know what Turkish law says about scapulars or crucifixes, but I assume it's just as strict.
America has a secular government, but that government must respect the right of the people to worship as their conscience dictates. This isn't Utopia, and there are plenty of cases clogging the courts where decisions have to be made as to whether a particular expression of religion is protected or prohibited. It's frustrating, particularly if you are on the "losing" side, but I think people lose track of how great we have it here, nevertheless.
That was my original point. I think I failed to express it clearly.
The easy solution is to ban all of it.
I like the idea of the black magic marker.....