We do not do them during the day while at school, UNLESS there is an increase in cough and they are really fighting some nasty infection. And then, I have been known to go up to school with the vest (we have the smaller one).
Otherwise, we go treatments in the morning before school and at night. We try to add one in during the afternoon, after school, especially during the winter. During the warm weather, the girls are running, jumping on trampolines, swimming, bike riding, etc, that I let that fun stuff take place of an extra treatment. They have recently started using the Acapella, and it is easier to get one in right after school or even during school if needed.