If you're teaching over the summer, I think it's reasonable to expect students to take lessons over the summer. I always try to let parents know right up front that the student's brain atrophies over the summer, and they've wasted a few months worth of tuition by letting their kid lose three months' worth of practice time. If you're part time, it might not matter too much, though. I also let the parents know up front that it's my living, and that I need to fill spots, and that their spot might not be available again in the fall (it usually isn't). Almost all of them are very understanding and flexible. i count on my income being cut in half during the summer because of vacations, and so I take on a few extra students to help out--it's nice for college kids to get a little theory or jazz instruction as a head start for fall classes.
Have fun!
Topic: Saving spots over summer