The flying schools that I have worked at range from the 'full 1/2 hour briefing & walkround' to the 'get them in, fly them, kick them out'. I agree with decmax that 98% are one-off flights for birthdays etc. Now, instead of a briefing, I show them the a/c during the normal walkround, pointing out what all the controls do; get them in - point out the instruments, and at TOC, after a quick demo of effects of controls, pass it over the them for as much/little hands on as they want.
Personally I quite enjoy T/L's: I get to do the T/O & landing (the fun bits), can sit back and enjoy the view, and teaching is at a minimum.