Be firm but polite. The idea here is not to contradict the caller by saying he is wrong and you are right. This makes mathematical sense, and parents readily accept such a structure. Some teachers charge more for a 30-minutes lesson than half the hourly rate. They feel that since they must cram so much into a short time, they should be paid more for their effort.
Many teachers charge the half-hour rate for a group lesson.
If the group lesson is in addition to regular weekly instruction, add this cost to the monthly total. You'll regret it. Bi-weekly lessons upset your schedule and come with a lot of administrative baggage. Claiming a tax deduction isn't one of them. If your studio is full, your students are happy.