Molesworth

Probably best to err on the generous side and allow "St. Custard's" as an answer, but Molesworth and his school were not created by Ronald Searle. That credit goes to the writer Geoffrey Willans, who created Molesworth for a column in Punch and approached Ronald Searle to illustrate his stories when they were published in book form. Searle (according to Wikipedia) had grown disillusioned with St. Trinian's and was initially sceptical about another school–based project, but the Molesworth books proved very popular and quickly outsold the St. Trinian's stories.