I would tell your brothers (you’ll get valuable emotional support from them, plus will be making them aware should they subsequently need to have any delicate conversations with their kids) and then you can collectively decide whether/how best to broach with your Mum. You shouldn’t be left carrying this burden alone.
As someone whose extended family has been devastated by SA between family members, not to mention as a HS governor who understands the broad and far reaching implications such a breach of trust has (not just for the abuse victims and their families, but also their friends/classmates, the school as a whole and the wider community of which it is a significant part), I would be giving genuine consideration to sharing what you’ve found with the police.
Whilst much time has elapsed, we sadly know that historically, incidents such as this were often swept under the rug or ‘quietly resolved’ by the school/education board etc so as to avoid a scandal. The police may well already be aware but equally there’s no guarantee that any proper investigation was ever conducted and there may be victims who have suffered all these years with no justice and no proper support.
Your father may have lost his career but those victims lost their innocence and their ability to have faith in those who occupy one of the most trusted positions of authority a society has. If I thought that even just the knowledge their abuser was now dead would in some small way help them to heal, I’d be doing everything I could to make it so.
Finally, I am sorry that you’ve discovered this revelation about your father. I can’t imagine the maelstrom of emotions you must be feeling finding out such an abhorrent thing about a man you have loved and thought you knew.