I think I agree with susie. I really think the woman is over-reacting. The girl is not being asked to 'dress like a muslim', but merely to have the good manners to follow a dress code. I take it her mother would ask her to dress respectfully for mass? Where's the problem?
The charge was, as the letter states, voluntary, and parents were free not to give permission for their kids to go. So it goes down as unauthorised absence - so what? Unless the girl is a regular truant then it's hardly going to stand up in court.
Pity the child from one school who went on a visit to a mosque only to realise, when they got there, that she had forgotten to take her scarf. Only thing was, her father was the Imam!