Pure Hell of St. Trinian's and The Great St. Trinian's Train Robbery
I realised it'd been far too long since I'd last watched these films when I couldn't remember a thing about the plot of PHoST. I did remember the plot of TGSTTR but that's probably because, this being a colour film, it gets shown on television more often.
frandowdsofa is absolutely right: TGSTTR is by far the weakest of the four and suffers from the the fact that the girls are almost secondary to the plot. PHoST is much more in the style of the previous two and, I think, a little stronger than Blue Murder at St. Trinian's. None of them, however, are as good as the original.