I used to tell my pupils that the Inspectors would be arriving to inspect everyone, including the pupils, and that they would notice every misdemeanour, such as talking in class, copying from one's neighbour and messing about. They wanted to see who was listening carefully to the teacher and who wasn't paying attention. (This was perfectly true in a way, as they would indeed have 'noticed'.)

Classroom door opens. Inspector enters, complete with clipboard. Pupils rigid with sanctimonious attentiveness. Result!