Could you set up some form of escrow agreement, where the money is drawn down as the services are delivered, but the school knows you cannot renege an go elsewhere?
I doubt you could draw up a legal contract for that.
Also the reason the schools offer say a 10% discount on a prepaid hour block is because they use the money in their business, and because their finances are mostly shakey they are unable to borrow the money conventionally.
If you can borrow money normally (commercially) you would not give a 10% discount - it's a huge discount.
What I don't know is the value (to the school) of getting the increased customer commitment which comes from having paid up front. It might be quite significant. OTOH, IIRC from my PPL training, not that many people (of those who actually have any money, let alone have enough to pay up front) drop out
during the training. Nearly all drop out once they have the PPL, but the average school is hardly interested in those.