Not normal.
Not legal.
Not what you should put up with.
Make an MOR or CHIRP report.
If the school is cutting corners like that on flying, what corners are they cutting on maintenance?
I'd be looking for employment elsewhere ASAP.
Remember that most trial lessons are booked and paid for well in advance - Vouchers, promotions, red letter days, birthday present for dad etc. Thus the money is in the Bank. If there is no instructor available, then the customer/ student will come back another day because they have already paid.
One thing that will ensure they don't come back to your school for the rest of a PPL course is a poor trial lesson with someone who can't instruct.
Best of luck.
Regards,
DFC