The purpose is so kids can finish their lunch? Kids can't eat if others are talking? That has no logic.
Please note, I'm not standing up for the school having this rule.. but fwiw... we actually do this at our house sometimes! My dd11 talks literally *all the time* and she is extremely social. She is a kid who would talk her way through lunch and totally forget to eat. I'm not saying that's a reason for the school to put the policy in place, but from what I've seen, she's not the only kid who talks rather than eating during lunch at school - every time I visited the lunchrooms in elementary schools that my kids attended, the kids were all so happy just to be together and have the chance to talk that they weren't eating as much as they were talking, and I saw a lot of food get thrown away at the end of lunch. I don't have any mindset that my kids need to eat everything in their lunch, but I would pick them up after school and they would be *starving* and 9 times out of 10, still have quite a bit of their lunch left.
Soooo... not saying the school should have a rule in place like that, but I do see the logic in it!
polarbear
