It's so they can get rid of them while they still have a low enough mileage to be able to get a decent price. Which, from my experience in the hire car industry, will be more than they paid for the thing, plus 40G k's of income being thrashed by renters!
Did the OP have a written agreement with the school, if so why is he complaining about something he has agreed to? if not, then he has no leg to stand on.
In my early days I cross hired aircraft from private owners and clubs because I had to; never again, the attitude of people like the OP is the main reason why. I had a 30 day termination clause in the agreement which I would use if they started coming in and throwing their weight around or thinking they could talk down to my staff. They needed me more than I needed them.
