So I suspect they’ve paid the full amount for 3 years ago which enters you into a different contract with the school I believe
It doesn't.
So it may be the school is worried they would threaten to withdraw their children which would result in the loss of 3 lots of fees rather than 1
IM (extensive) E of independent schools, they do worry about this if they are smaller schools with lower margins. However, this worry tends to be outweighed by the worry of losing several more pupils as a result of being known to have failed to stamp on bullying.
It is shameful that your DD's school has done nothing about this. Girls can be particularly good at bullying 'under the radar', and schools ought to be aware of the ways this can happen. Is it a girls' school? Not that it makes any difference per se - just wondering.
Have you discussed this with other parents? Schools become very jumpy about stories being circulated which harm their reputation. One close to where I live has been known to try to take people to court for telling the truth about them spreading rumours.
I'm generally pretty peaceful and disinclined to rock the boat - but I would be very tempted to take your DD out, even at this stage (same age and stage as my DC2). I would also explain why, to the school. I would also refuse to pay for the summer term, on the grounds that they are not fulfilling part of their contract with you, namely to keep your daughter safe.
Have you taken it up with the governing body?