Many of us on PPRuNe say it time and time again: Don't pay large amounts up front unless it's into an escrow account or equivalent! Although I think paying small amounts up front is a reasonable compromise. Say, not more than a few grand but never more than you're prepared to lose if/when the school shuts shop.
I've lost track of the number of schools that have gone belly up. Australia, UK, USA, you name the place it makes no difference. The students and staff are nearly always the last to know, in many cases only finding out when they arrive for lessons or work and find the doors locked.
There are schools where paying it all up front would be OK for the period of your training. Trouble is you have no way of knowing which schools are which. You certainly don't have the option of auditing their books or discerning the owner's intentions.
I can only repeat the US's (futile) anti-drugs mantra: "Just say no." As Scroggs so rightfully said, if the school won't compromise then go elsewhere! They're the ones begging for *your* money. You are not the one begging for their services. You can *always* go elsewhere. Just as much as asking about their rates, aircraft availability etc it's important to ascertain payment options. A requirement to pay large amounts up front should raise a very large red flag about choosing that school.