Indeed, Bingley, there were four films in the series: The belles of St Trinian's, Blue murder at St Trinian's, The pure hell of St Trinian's, and The great St Trinian's train robbery. Halliwell mentions that the first was an enormous commercial success, but the sequels went from bad to awful.

As I've just been given a boxed-set of the four titles on DVD, I'll be able to judge for myself (when I get the time to look at them!).