ENID BLYTON - The Naughtiest Girl Saves The Day

In Enid Blyton's bestselling school series Elizabeth Allen is sent away to boarding school and makes up her mind to be the naughtiest pupil there's ever been.
 
 
 
In book seven, the school's strawberry plants are vandalised and a lost blazer button at the scene of the crime leads straight to the Naughtiest Girl.
 
It is hers, but how did it get there - and why would anyone want to pin the blame on Elizabeth?