I looked into this a year or so ago...my understanding of how it works is......
You pay to get qualified...then put you in a franchise in your home town or very near to it.
I think then.....you can either advertise yourself to generate clients or you can pay them (the driving school) a %age of your hourly rate or a flat fee for a referral to you for a lesson!!
I have a few clients who are driving instructors and some earn over 30k some less. It depends on how much money you want to earn and how many hours you want to put in.