There's no legal minimum or maximum temperature
Schools should follow government guidance on workplace temperatures, which requires a 'reasonable' indoor temperature. For classrooms (i.e. places where people aren't doing physical work) the guidance says this is 16ºC.
There's no guidance on maximum temperature, but your emergency or critical incident plan must cover 'severe weather' - for example, heatwaves.
You have a legal duty to make sure classrooms are a 'reasonable' temperature
This is set out in the Workplace (Health, Safety and Welfare) Regulations 1992 (section 7).
The Health and Safety at Work etc. Act 1974 requires employers to ensure the health, safety and welfare of their staff and others. Classrooms and other school spaces that are too hot or too cold may put the health of