With regards the earlier comments about paying up front. Many of the schools will offer discounts for pre-paid packages. These can be worth looking at, but often exclude tests costs and include only the minimum hours. The pre paid packages can benefit you when the price of fuel rises.
Essentially the biggest problem is that every year a few flying schools seem to go into liquidation.
The only way to prevent yourself being at financial risk is to pay using a credit card (or a visa debit) who will refund you should anything happen. I once lost about 600 hundred pounds when a flying school went down, I called my credit card company and they refunded the entire 1000 pound initial payment that i'd put down.
When choosing a school, it's important to find one thats accessible, with instructors that the student is comfortable with and where they feel happy learning.