The simple fact is, that you as a punter have absolutely no idea of how financially stable a flying school really is.
Dead easy.
If they don't offer any pay-up-front deals they're pretty stable.
If they offer a pay-up-front deal which works out at an interest rate higher than a sensible bank loan they they're dodgy as hell, because this means that they've already tried to borrow from the bank and been turned down for being too risky. If the bank won't lend them money what makes you think you're better placed to judge the risk?
The only case that isn't immediately obvious is a school which offers a very slight discount on pay-up-front deals, so that they're paying you slightly less interest than they would have to pay on a bank loan, and you're earning slightly more interest than you would earn on a bank savings account, ie the deal is set to split the bank's mark-up. That
probably means they're
likely to be OK, as it suggests that the bank will still lend them money if they need it.
Of course ... this is pre-credit-crunch conventional wisdom, and relies on two basic facts of financial life which may no longer be true:
(1) banks are a better judge of risk than you are
(2) banks lend at a higher rate than they borrow.