Because there wasn't enough time in class to do the work set out? Not just dd but quite a few of the others in her class too. This is not the first time they have been made to stay in its happened a few times.
I don't really mind her missing the odd playtime as I think it does them good to see actions have consequences and the school need to disapline children somehow BUT I think lunchtimes are a bit long and children need that longer break to make sure they get enough to eat and have a good run around so they can sit back down and concentrate in the afternoon.
What do you think, what would you do?