Oh , that's lovely indigo 
Do you really not remember being 15 years and doing something you now feel was silly or immature, ill-advised or even dangerous.
I actually feel it's a bit 'slut shaming' [not that I am saying this girl is a 'slut', I actually hate that word as it is in the main used to describe female sexually promiscuous behaviour and not male].
Oh, how awful that a 15 year old dares to wear a bikini top and a pair of shorts and then puts a picture of themselves on facebook... Really? Yes, the girl in question should have a lesson on security settings but otherwise it shouldn't really be an issue imo. It is normal dress within a certain context and should not be viewed as sexually provocative imo.
To my mind, this incident has been a huge betrayal of trust implicit in the teacher/student relationship. Irrelevant that the girl is in a bikini, why did they have to use any photo of hers which a] was identifiable and b] without her knowledge and permission?
There are much kinder ways to make the point that online material is easily accessible than singling out a 15 year old pupil in her school assembly.
Very bad judgement on behalf of the teacher, how do they know that said pupil is not at risk of self harm for example as a result of being humiliated in this way?
If I was the Mum I would take this issue as far as I could within the official channels also.