I dont see the problem in parents lingering. This social connection is extermely important if you hope to make more contacts and hope to have a good student relationship. The amount of times I have been asked out to families house for dinners, special events, it is very nice. I like this side of teaching very much and if you just treat teaching as an efficiency system awlays watching your watch and never spend extra time with students etc, then you get a plain teaching world thrown back at you. If you feel people staying around you is a problem then you can't be enjoying their company. People can feel that very easily. $ is not everything, being late for the next lesson by 10 mins is nothing, making people feel less important is devistating.
If you are tied up for time you simply say that. I have had to apologise to parents who want to talk and talk after lessons because of this and they don't mind it at all. If you are sincere and sorry for not being able to spend more time today, they will understand, but if you never offer them extra time, some personal talking time with you, then you simply become a teacher and nothing else. Pretty plain and boring if you ask me. A student is absoluetly delighted and will never leave you as their teacher if you give them extra time in lessons, whether that is to talk about music or other stuff.