The Workplace (Health, Safety and Welfare) Regulations 1992, Regulation 7 (1) states: "During working hours, the temperature in all workplaces inside buildings shall be reasonable." High temperatures ~ Heat Stress and dehydration can be serious problems at temperatures above 35 degrees centigrade so that should be regarded as the maximum reasonable temperature for prolonged periods of time in school classrooms.
This temperature is above comfort temperatures but healthy children should be able to cope with this if they are given generous supplies of cool water to drink. More sensitive children may experience problems at much lower temperatures and staff need to watch for signs of heat stress at temperatures above 28 degrees centigrade. Schools with inadequate supplies of drinking water need to make arrangements for children to drink enough water.
It should be remembered that the amount of water that can be delivered through drinking water fountains is small. Minimum temperatures ~ these are specified in the school premises regulations. 18 degrees centigrade is normal for classrooms. If this is not what you are looking for or if you want more information you can contact the Department by email at info@dcsf.gsi.gov.uk or telephone on 0870 000 2288.