Why was the teacher opposed to fans?
What sort of lights were there in the classroom if turning them off was going to make such a difference?
This sounds like a teacher who needed to use a lot more common sense than he or she exhibited.
My DCs went to school in the US, where they enjoyed AC in all their classrooms. In their (private) elementary school they had window AC units, and in high school they had central air.
We live in an area of the US that has brutal winters and hot, humid summers. Kids get out for the summer at the end of May and return to school around the third week of August, when the heat is truly oppressive. Not having AC just isn't an option. It's not considered humane to expect either teachers or students to function in either extreme heat or cold.
Extreme heat events are going to happen more frequently. The UK stands pretty much alone among developed countries in inflicting a summer term on students and teachers, and will have to take the finger out and think carefully about the reality of the working environment if school is to go on as it is now.
It's really stunning and very saddening to see the acceptance among British posters that there is no money for heat or cooling, that everyone in schools should suck it up and soldier on. This is acceptance of second best or even worse as normal.