I can see a few valuable things about after school detentions:
- Combatting refusal to do work. The thought process being "I might as well spend 20 minutes on my homework now rather than end up in an hour long after school detention tomorrow where I'll have to do that same homework task anyway". Obviously it doesn't work everytime, but I have seen many occasions where a warning of "If you miss one more homework, then you'll be completing it in detention" has caused a homework to come in on time that otherwise wouldn't have.
- Demonstrating to other people in the classroom that certain behaviours won't be put up with. Once again, even if it doesn't necessarily stop the student being punished, if other students see one kid getting away with murder, they might be more inclined to join in with that negative behaviour, because they don't have any consequences to fear. Though to be honest, I think in situations like this, removing that student from the classroom altogether is the best solution, if it's got to a point where their behaviour is seriously affecting others' learning.
- Opportunity for a teacher to have a private word with a student. Even if you step outside to have a word with a student about their behaviour, their friends in the classroom are very aware of what's happening, which might encourage that student to put on a bit of a front. In a detention setting, where there's nobody to 'perform' to, some students behave very differently.
- Opportunity to catch up on work that was missed due to poor behaviour. If a student was messing around all lesson and only completed 1/4 of the required work, having them in for detention to complete the rest of the work ensures that they don't fall behind.
Now, I don't agree with just bringing the same kid into detention repeatedly. I think after the third detention, higher ups should be getting involved and other interventions should be put in place to deal with that poor behaviour. I'm also not a fan of hour long after school detentions, I think short detentions at lunch / after school are just as effective for the above mentioned reasons.