That really depends on your locale. Since you mentioned the temperature in Celsius instead of Farenheit, I know that you don't live in the United States.
If you would be kind enough to post where "school" is, then one of the experts may have a better chance answering your question.
I will say, however, that 35 C is in the mid 90's from a Farenheit perspective, and we had to go to school even if the temperature was that high.