The school has a duty of care towards its students
If they discharged that duty, they are not responsible (liable); if they didn’t, they are.
The duty is discharged by acting reasonably. Without knowing what the school did or did not do and how, if at all, that contributed to the injury, it’s impossible to guess if they met that standard.
This is the same as the negligence standard that applies to everybody. The only difference with a school is that the duty always exists towards students and the duty is non-delegable, that is, even if control is passed to someone else (e.g. the bus or venue operator) the school still holds the duty (as well as the delegate).