I disagree that the bullies shouldn’t be held responsible for their behaviour.
Withholding public criticism doesn't mean that the school can't hold them accountable for their behaviour.
If the worst bullies (such as the ones who spat on the girl) were suspended, they’d have space and time to think about the dangers of mob mentality.
The school plausibly would not be able to do that without attracting legal action from the parents if it were to interfere with the girls' exam prep or their exams and any offers that they have.
I find it hard to believe that a school leadership team would deal with this incident so appallingly.
I would second a decent investigation of what happened here. It sounds horribly akin, in many ways to how the adults in the Child C incident abdicated their professional and adult safeguarding responsibilities to that young woman.