Fair number of answers, no one saying what country they are from
US here, we had to ask but it was basically always a yes.
Reasons for a no:
Middle of a test
Middle of explaining something important, then you might be asked to wait a few minutes
If you were just abusing the system asked always asking just to get out of class they would probably start saying no. If you had a medical issue you could always meet the teacher after class or before school and say you have a medical issue that prevents your from waiting. It it was a sudden thing like diarrhea… Well just get up and go.
Want that the story about the teen aged alien that came here to jack off?