The Sixth Form at St Clare's: Book 9 by Enid Blyton

Head girls at last, in the series' final chapter

Pat and Isabel O'Sullivan close out the St Clare's series as newly appointed head girls, tasked with keeping order in their final year at school. Their authority is tested when sixth-form advice meetings meant to help the first formers are twisted into a tool for bullying and blackmail by a manipulative classmate. Written by Pamela Cox as the concluding book in the series, it ties up long-running threads with genuine emotional weight, giving the twins, and longtime readers, a fitting send-off from the school they've grown up in.