It's too sexualised.
In an ideal world, there would be a discussion about why it's inappropriate.
I can imagine the school might not want to make matters worse for the girl in question - but a culture where pupils know sexual harassment is utterly inappropriate has to begin somewhere.
All those who think this is a joke - would you tolerate this from work colleagues?
No, of course you wouldn't.
And all those saying that it's just children's humour - how do you expect 15 year olds to move from the point where they think this is OK to the point where they know it's inappropriate?
The answer is by being told.
And that, of course, comes down to being told that a given space (workplace or school) doesn't accept this behaviour within it.
The thing about an image like this lies precisely in the use of a sexualised image negatively, for harassment purposes - in order to make the person picked out feel vulnerable and mocked - sexually.
And that, of course, means that every female in the school is now mockable/harrassable on the same, sexualised, sexist grounds.
If I were a female teacher in that school, I would feel deeply unhappy with that image being found and dismissed.
Because I would know that the school doesn't take sexual harassment - on grounds of sex, by means of sexualised imagery - seriously.