The Muslims and the Sikhs can, but not the Christians. I know that suits you just fine, and some people agree with you, but rational people don't.
It looks like you're the one who doesn't the article
Lawyers for the school denied discrimination and said the purity ring breached its rules on wearing jewelry. They said allowances were made for Muslim and Sikh pupils only for items integral to their religious beliefs and that, for the same reason, crucifixes were also allowed. But it argued that the purity ring was not an integral part of the Christian faith.