I don't claim to really understand what the purpose of state atheist policies is, perhaps someone can ELI5 It me, but at least It seems fair honestly, as:
The school is part of the GO! network, which has a general ban on wearing religious symbols.
If it was a ban explicitly on Muslim headscarves it'd be discriminatory. Another question is whether this will be enforced in equal measure on a white kid wearing a cross though...