First off, if you can find a school who'll guarantee you 500hrs a year, then they will soon be bust!
There is no way I'd agree to a commitment like that for even a busy club like ours, especially with something like a ropey old cess-pit.
Buying an a/c to lease to a flying school in order to earn money is a complete non-starter. Leasing may help you to offset some of the costs, but unless you are a maintenance organisation, then there isn't any money in it. Long term there may be, but there can be some disasters along the way.
A couple of our machines are leased in this way from private owners, but they certainly don't see it as a money making scheme and I have made that clear from the start.
One a/c we have we lease with a commitment of 30hrs a month (360 per year) not particularily onerus, but during the winter it can get pretty interesting!