For me, the answer is that it depends. For the aircraft that goes faulty at location B, I would expect the school to pay. After all, you are renting the aircraft of which they are carrying out the maintenance and making sure it is in an airworthy condition.
Regarding paying when the aircraft goes wrong during an exercise it would depend what stage I was at. I was at the end of my training and to complete certain exercises to finish and having to duplicate it again then I would be a little bit hacked off. If I needed more hours anyway and would have had to flown that time then I would put it down to experience flying as it wouldn't be costing me anymore money through no fault of my own.