So I had to start a new policy: you have to have a note from a grown-up to stay in. It was working pretty well. I could just say "where's your note" and they'd give up and get back into line.
One day, we were already 2 hours into school and just about to go out to recess. One of my girls walks up and pulls a folded 2-inch piece of paper out of her pocket. She has it to me and says, "Oh, this is from my mom. I forgot to give it to you earlier."