The director Sue Dunderdale had taken Mikey North and Jessica Harris, who play schoolkids in detention, on a research trip to Fortismere school in north London.
While there, they ran through a scenes in a corridor that involves a lot of swearing. Out popped the head who, thinking they were his students, thundered it was "not the type of language he wanted used in the school".
They only just managed to escape detention.
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