In fairness the website does make the following disclaimer:
"A delightful and very interesting pen and watercolour on paper laid on board of a St Trinian's schoolgirl. Signed with initials R.S. and inscribed in pen on the board "Ronald Searle". My local auctioneer has decided that without provenance they would have to catalogue this as either "after Ronald Searle" or possibly "Attributed to Ronald Searle" - hence my decision to sell this on Ebay with a low starting price and NO reserve.
This was bought at a market in Brighton as part of a folder of mixed art work which included a number of pen and ink drawings by different artists (none of the others unfortunately by such well listed artists as this one)."
I don't believe Searle made any St. Trinian's drawings in colour until much later on in his career and he certainly wasn't signing just his initials so late. I think even by the late 40s he had dropped the R.S. & was signing the St. Trinian's drawings with his full name.
The hair seems rather clumsily defined-Searle's curls are much tighter than that. That area above where the cheek is resting on the hand is quite poorly handled.
It's most likely a piece made for fun with perhaps the character & its pose copied from Searle. Even the ink splatter effect lacks the 'control' that Searle had. What do you think?
Contributor Stephen Nadler sent in a scan of the original St. Trinian's cartoon by Searle for comparison. See his analysis in the comments section.
"I'll just die and then you'll be sorry."  Ronald Searle's Golden Oldies, p. 131
Here is a very early version that Searle must have re-worked
I have several examples where Searle returned to a gag years later or re-worked the same drawing. Should make for an interesting post at a later date. I also have some video clips where Searle based animated shorts on earlier print cartoons.