Bonnie the dog was taken to Ilfracombe School and Community College where she was patted and stroked by the students during a drugs talk.
But as she left the classroom, Bonnie became jumpy and picked up a scent she didn't like in the lockers.
Police warnings
Bonnie was allowed to search the locker room, where she allegedly found the drug.
One 15-year-old schoolboy, who was charged with possession of cannabis, is appearing in court as a result of the find.
Three other pupils were given warnings by the police.
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