And I think parents should PARENT and insist their child wear a coat when it's 12 degrees outside. And, no, yesterday and Monday they wouldn't have "cooked" at receess time given it got up to a spanking 20 degrees by mid day. (also, you're lucky if your kids go out for 30 minutes for recess every day - most don't get it on cold days)
Yes, on a 40 degree day it's fine to be without a coat. On an 18 degree day it's not.
So you can make a rule (and you can PARENT and insist they wear it even if they tantrum, turn red in the face, scream, throw themselves on the floor, and end up sweating in said coat) that if it's less than 32 degrees (as read by an impartial thermometer posted outside the window in a shady area of the house) then they must wear a coat and hat - mittens/gloves optional. Period.
My mother did that when i wanted to wear my poncho (oh, how I date myself) - I wanted to wear my poncho when it was 5 degrees outside because it was so lovely, and had pom poms on the end of the tassels.... and swirled when I twirled.... she said you can wear it any morning when it's 50 degrees or higher outside. I learned to read a thermometer very well that very day. I would check, and either wear it or not. Ah.... I loved my poncho! And I was in 1st grade - this rule continued through 4th grade when I outgrew my yearly new knitted poncho from the neighbor.