Whilst I concur with Pauls sentiments to a certain degree, I would also suggest it depends on who you are training with.
I usually fly out of Redhill and I tend to make block payments, partly because when I have some cash I give it to the school so it is ringfenced for flying and also because I get a slight reduction in costs.
It would not cross my mind that the operator in question would not honour my payment, maybe I am nieive.