Issue One - the 'purity' ring is not a religious symbol, and comparing it to a hijab is patently ludicrous. To suggest that it is a 'Christian' symbol is to allow a child to confuse a moral stance with a religious emblem, and her mother should be back the school's health and safety policy, which is for the protection of all pupils, explain to her daugher the difference between a pop-influenced fad and a fundamental expression of religious faith, and stop making a fuss about very little.
Issue Two - the point is not whether the sanction is unduly harsh - a matter of opinion, but the fact that the issue should have been dealt with internally, which is a matter of common sense. In these PC times, it's always best to leave beaurocracy out of the argument until the last possible minute - and then leave it out anyway!