The Naughtiest Girl: Naughtiest Girl Marches On: Book 10 by Anne Digby
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. Rather than report it, Elizabeth plans to investigate - and, with the help of some friends, unleash a surprise waterpistol attack on the boys. But Elizabeth has been set up, as she discovers when she runs straight into their form teacher and the head boy - mistakenly squirting them both with water. Now, Elizabeth is in trouble again - and she could lose her prized monitor's role. Can she find out who has a grudge against her - and why?