Naughtiest Girl: Naughtiest Girl Marches On, Paperback/Enid Blyton

Elizabeth is overjoyed to be appointed monitor again, especially when the new head-boy and girl have such exciting ideas. But one of the second form boys is slowly turning all the other boys against her, starting with a nasty note in her desk and ending with a false tip-off about a midnight pillow fight in the boys' dormitory. Can she find out who has a grudge against her, and why?