Duchess,
I am not going to be drawn into any argument here, particularly as I've already pointed out that CofA etc. will be payable irrespective of whether or not the student turns up. Any half busy club will soon make back the lost money on the aircraft from that couple of student hours flying, even if not on that day, certainly by the end of their financial year. For a start, they could do a block hour building deal with a newly qualified PPL ....
I think you are missing the whole point of this thread, Solar stated as part of his question:
I don't know how legal it is and how could it be enforced anyway
What I am saying is that from a legal point of view, if Solar didn't pay the penalty fee and was taken to court, there may be scope for a half decent lawyer to put forward an argument and potentially have the penalty reduced or declared unenforceable depending on circumstances.