It's a difficult one this. Having been stood up by many students, it is absolutely-mind-bogglingly frustrating, especially for a double slot on a saturday morning.
The excuses have been laughable, the worst was when I rang a particular stude, only to be told that his wife wouldn't wake him as he had been out on the town last night and had got in at 4 O'clock steaming drunk. I hit the roof.
We have never had a policy of charging for no-shows since I would feel uncomfortable about not giving refunds if the a/c was unavailable. So in the interests of fairness we made the decision not to charge.
We will have a word with any transgressors about not turning up and if they continue, then we simply ban them from the school. This has only happened once to my knowledge. (The chap who's wife wouldn't wake him and that was because it happened on a large number of occasions.) If they turn up late, then they simply get a shorter lesson so as not to push the program out and inconvienence everybody else.
Most places will bend over backwards to try and get you flying if possible, but it can sometimes be beyond our control. In that case I'm afraid you'll just have to lump it unfortunately. It's not the best result for either side really.