I am an agnostic non-christian and usually in the side of being politically correct, but I can't really see why she can't have a cross pendant. Thenagain I don't see why student's can't have facial piearcings in some schools. Some snobbish better folk's schools in Japan even prohibit dyed hair.
But arguing that putting the cross under her shirt is "denying Christ" is a bit extreme. I would imagine a person who can't put away a cross couldn't always be an objective teacher when it comes to telling kids about cultures and religions of the world and then there's (safe) sex...