That sounds like bollocks to me, often the instructor will monitor more than one student at a time when they are solo. Are you a ppl training for cpl or just training for your ppl, I flew with many different schools and this is crap, don't allow it to happen, you are the customer.
While the instructor is on the ground he does not spend one hour doing nothing but monitoring you, he gets on with paper work and general tasks so you should not pay for this. There are many flight schools, so don't be strong armed into crap service.