Hmm, well I'm going to give a slightly different view:
Sending bitchy messages about people on social media, then using that social media on 'friends' phone and then giving it back to the friend without closing it down, sounds like a BIG mistake to make and one your DD is sadly paying for now.
It will be very hard and painful for her, and I'm sure we can all sympathise, as if people heard what I said to my DH about them occasioanlly it would be mortifying.
She's had her private bitching exposed, through a mistake she made, and now there is a fallout.
Asking a friend to close the account sounds like a 'filler fib' to minimise her own negligence tbh. I bet she just handed it back and forgot.
Yes in an ideal world other people should have averted their eyes to the messages when they saw it was her private account. But it's not an ideal world it's one where people are curious and nosey, particuarly once aware there was information there about themselves, you'd have to be the Dalai Lama to look away.
So a mortifying situation.
To the HOY this looks like an unpleasant row between girls created by social media. I can understand they'ev said not to discuss it in school to try to prevent it becoming any bigger.
If girls are spreading this row to other groups then yes HOY needs to deal with that directly.
I bet however that every parent of every girl in this group believes their DD to be 'wronged', and every parent will a story to illustrate 'how upset they've been.'
Every parents of a DD in a mean girl group sees their DD as the victim, but usually from outsiders perspective the victim moves around the group at difefrent times and girls join in with the ostracisation when they're not the victim. Unpleasant but true.
If these girls have form for this kind of fallout/drama/unpleasnatness I would be focusing on heling your DD to come to some decisions about who she wants to be friends with, what kind of person she is, and the kind of people she wants to associate with and be part of.
Help her weather the mortifying fallout from her mistake, and in the process move on from this nasty dynamic which she is currently choosing to be part of.