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Thanks for your answers, I spoke to another parent who had inquired with the school around what details they actually recorded about individual detentions.
They have some set procedures which teachers are instructed to follow, such as recording that a child has attended a detention and what they were given to do during the detention. If a writing assignment is given as in my sons detention, teachers are instructed to check that the writing assignment has been completed to a satisfactory standard and to then dispose of the assignments by tearing them up as they are not required to be kept and they are of no further use. If the teacher considers that the assignment has not been completed to the best of ability, they can ask for the assignment to be re-written.
This seems like a fair procedure to me, though I 'd be interested to hear if anyone else agrees or has different views.