You may remember that the BBC had to pay around £2M to Sir Cliff Richard a few years ago after revealing that he was under investigation for sexual assault. Some of the press and social media would have you believe that this judgement broke new ground legally. It didn't. It reiterated the long standing position that everyone has the right to privacy. This means that information that someone is under investigation can only be revealed if it is in the public interest (which, as the BBC forgot, is different to it being interesting to the public). Unless the school was asked to reveal the information to parents by the police, they would have been on very dodgy ground by doing so. If the teacher had not been charged, they may have had to pay him a significant sum in damages.
It is true that the information was in the public domain once he was charged and the case went to court, but then a different set of rules kick in. The case was then sub judice, which limits public discussion in order to ensure that the case is not prejudiced. The press understand the rules and have additional protection that is not available to others. If the school proactively sent information to parents at this stage, the head might have ended up charged with contempt of court.
The authorities have to be very careful with cases like this. Whilst publicity may result in additional victims coming forward, it also increases the likelihood of false allegations. That applies particularly when the victims are young children. There are a number of documented cases where well-meaning parents have unintentionally implanted false memories of sexual abuse. This leads to false allegations against the suspect which can end up weakening the case (if the defence can show that some allegations are false, the jury may be less inclined to believe the others). It also leads to unnecessary trauma for the children - similar to the trauma if they had actually been abused. As per my previous post, parents should not question children at all.
The sentence he has received is what I would expect for these offences. Additional victims probably wouldn't have made much difference to the sentence. Whilst the offence has a maximum sentence of 5 years, the details of this case do not put his offences into a category that would have led to a prison sentence.