As
hemac said, it depends on the school. It does also depend on
some of the factors mentioned by
Pompey Paul.
Firstly, if the school goes bust, the credit card company should cover the amount. Secondly, always get your medical before you commit to any flying, paid in advance or not. Thirdly, the OP has had a few lessons already so therefore knows whether he likes it or not or is likely to get airsick. Fourthly, work and family commitments are the OPs business and only he is in a position to know whether he can devote the necessary time. Let's assume he can otherwise he wouldn't be asking!!
The points about fuel surcharging, aircraft serviceability and availability though are valid and enquiries should be made about these scenarios. If the OP decides to learn full-time, then the risk of exposure is very much lessened. Similarly, aiming to fly several times a week will also lessen this risk.
I paid up front for a PPL(H). Shock, horror, what was I thinking of???
I did my PPL(H) full-time with one of the most respected operators in the business. I carried out full Dun & Bradstreet checks (they got an A1) and obtained the last two years worth of Financial Statements. Therefore, my exposure to £15k, full-time was 6 weeks, a risk I was prepared to take for the discount offered. As
lauchiemb says, it's not all about businesses going down the pan; this operator had cash in the bank; no overdraft. It's about business and commercial decisions such as customer loyalty and retention.
Cheers
Whirls