Well, if they offer a substantial discount, then that's a pro. But the cons are that you are likely to lose your money if you get into a problem while flying (eg. medical), get into trouble with the school, or the school gets into trouble. Lots of people have lost serious amounts of money that way.
You might want to find some way of getting some additional security. For instance pay the sum via credit card. If the school goes bankrupt, you can claim your money back via the credit card company. Or negotiate with the school that your money is put in escrow or something, to be released to the school only after each lesson, and to be released to you only if the school defaults or if you lose your medical. Or something.
If the discount is only a few percent, I don't think I would bother and just pay as I go. If the discount is substantial, well, read the contract carefully, see if you can do a check on the financial status of the school, see if you can obtain some extra insurance and go for it.