Check what you are being charged for:
Is it wheels off, wheels on - i.e. time in the air.
Or wheels on/wheels off plus 10 mins (say) to allow for taxying and holds at a busy airport.
Or, at the extreme, as one flying school - long closed - that I know of, was charging: master switch on to ms off. And they made students switch on the master switch before doing the walk-round!!
(ps. They still went bust. Overheads higher than income!!!)
And then there are landing fees.
And make sure you are getting a proper brief before each lesson - and how that is paid for. (often: oh, don't bother, we'll cover that in the cockpit, let's get flying!)