What about some positive reinforcement, or good old fashioned bribery?
Get something inexpensive and easy that the kids all want - some shiny glittery stickers, a hand stamp, or a small prize of some sort (oriental trading or a party supply store are great for things likes this that are often ten cents each) and do a "treasure box" - then if they follow the rules all day, they get a special sticker, prize, etc. right before pick up. I had to do this with my older kids, only since they're not little I make them wait until Friday to choose a treasure. It works like a charm - and the odd man out who doesn't get a treasure usually tries harder the next time, because they see that everyone else got a treat and they did not. Ever since I started this, my grade schoolers are so much better at following the rules, and all I have to say is "who's not getting a treasure on Friday?" and they all straighten up immediately!