It's likely you're thinking of one of these books by Enid Blyton:
* "The Naughtiest Girl in the School" - This is a popular book in the "St. Clare's" series. It follows the adventures of Elizabeth Allen, a mischievous but ultimately good-hearted girl attending boarding school.
* "The Naughtiest Girl in the Class" - This is a book in the "B.B. (Big Book)" series, featuring the character of "B.B" and her classmates.
While "Naughtiest School Girl" is not a title by Enid Blyton, it's easy to see how you might misremember given the common themes of schoolgirl mischief in her work.