Some school's do seem to try this trick. If there are no headsets about, you'll have to buy one, but what happens when you take Mr/Mrs trial lesson flying and they want to take a passenger?! Believe me it does look bad when you have to go scratching around for another pair.
Explain this to them and see what they say if the answer is "buy 2 headsets then" look for another place, 'cos if they moan about £100 what state do you reckon the a/c are in?