Years ago, I had lessons scheduled back-to-back. I had to be able to switch gears very quickly, and I was able to keep those lessons streaming successfully, if I do say so myself. (If students were late, I could not hold them over, and they learned to be on time.)
When parents and I set up the lesson time, I explain that I will have to start on time, because another lesson begins at whatever- o'clock. Remember that they are paying you for your time as well as for your experience.
Of course, I was younger and quicker then. I am not so strictly scheduled these days, even though I still teach many students. (Sometimes a 45-minute lesson throws the clock off by 15 minutes for scheduling, and now I am grateful for a 15-minute "break". )
So I have learned to avoid the long gaps in between lessons, but I am am still trying to get the kids out at the end of the lesson when there is not another lesson immediately following. They (and their parents) linger. A compliment to me, I am sure, but it eats up those few little 15-minute spots that were to be mine, all mine.