Again think who told you this fact and who in the past have been proven to be liberal with the truth.
Also as well if you really do get someone that is hopeless its very very hard money that you earn off them and you don't make as much if any profit off them.
Also as well you need a constant stream of punters, you can't have 50 turn up on your door when you only have 10 aircraft and 15 instructors ( I don't have a clue how many they have) So you just take the best of what turns up on the day to fill all your slots be that 30% or 50% for that selection. If a previous course is over running you may take less than that. Which is where the difficulties come in. Also this allows you to charge a premium for what the punter thinks is an elite product when in relality it is no different to any of the others.
If your staff are salaried you have a set fixed salary costs. If you worked out that you need to get through 100 folk in a year at X amount per head and you only are able to shift through 70 your unit cost per student has soared. If some of them fail to complete you also have spare assets sitting there which need paid for hence them wanting to crucify you if you leave.
Apart from which if you charge for the selection day its quite a tidy little income stream for not alot of work.