Sorry I don't have an answer for you, but I would like to comment on delboy's and annie's remarks - I think there IS a safety issue here. I am more than happy for my 12 year old to walk home from school at the same time as her mates, as there is an element of safety in numbers. Half an hour later? Not a chance. Unfortunately we do not live in a safe world sometimes, and I agree with wizard that break/lunch time detentions would be more suitable.
Also, what if you had after-school commitments or appointments? My children are at clubs (Cubs, Guides etc) most nights and I'm at college, so I wouldn't be best pleased if my daughter wasn't allowed home when expected.
And before you all have a go at me as well, I would be the first to support my children's schools if they had cause to apply any sanctions - I just think that after-school detention is out-dated and ineffective, besides potentially dangerous.