To take it or not to take it?

This morning, the dean of academic affairs asked me if I could teach the third-year students again next semester. I told him that I had to think it over for a few days. Hmm, I'd like to help, but I've promised a practice- teacher-to-be to be her supervisor. Besides, I've planned to take a trip this summer vacation. Here is a dilemma.

The dean told me if I agreed, he would arrange for another teacher to supervise the practice teacher. So this seems easy to solve. But the summer class...