Old fella,
you're pretty brave if you think having an untold number of gift vouchers out there is good. Sounds more like a liability to me.
Our gift vouchers all have a 12-month expiry. If the prospect is keen, they will use it within 2 months.
Otherwise, it is a novelty gift with a <10% chance of turning into a student.
If someone comes to me with a gift certificate up to 2 years old, yeah we'll usually honour it but then, we're great guys and country flying instructors like you.
That school must really dislike the prospect of tens of thousands of dollars in additional revenue.
Assuming for a minute that a school makes $100 profit per hour of flight training, that's $4000 profit per PPL that goes through to completion.
I would be surprised if many flying schools are making profits like that, I know we certainly aren't!