This seems to me to be a case of someone trying to get away with reverse discrimination.
Her mother seems to think that because they are religious images they should be allowed to remain even though they are not part of the uniform.
She should have been suspended and she should be kept out of school until she conforms or they change the school rules and allow not only her religious icons but also the skull and crossbones and band badges.
The problem then of course is how much of her education she is missing. But if it doesn't concern her mum too much I'm happy to let that be her decision.