I had a colleague who NEVER answered or sent any emails. He worked on the principle that 95% of the emails he received were just copies of messages he had already been given orally and that the remaining 5%, though urgent or needing a reply, would be asked of him in the corridor in a “hey Jim do you have the [insert topic here] I asked you about” to which 99% of the time he would say yes, give an answer orally and continue his day. He reckoned that it saved him 30 minutes each day of trawling through his emails and, most importantly, he only told people he did this after he retired and no one had ever realised.
90% school email is unimportant or is information that could be shared orally or via the staffroom notice board.
I’m not suggesting this to be THE answer but it is AN answer!