I looked these up, and they are Sikh bracelets, not Muslim.
The Muslims are wearing other religious symbols or garb.
Where these things, wrong as they are, are enable is due to the fact that Christians are not commanded to wear or otherwise display any outward symbol of faith; we ARE the symbol of our faith, by application of our faith.
Indeed, to feel that one must wear such a symbol, or lose their Salvation, is a form of idolatry, that is no different than “going whoiring after strange gods”.
Our salvation is not in things, but in acceptance of Christ’s Atonement on the Cross for us. The school can never remove that from one’s heart.
Where is that in the Bible? The closest thing I can think of is Jesus' command to not pray like those hypocrites who go out just to be seen; in other words, out of pride.