Just a couple days after jet-setting from France for the Cannes Festival, Sarah Jessica Parker was back in NYC in mommy mode walking son James to school. Both in Hunter Wellie rainboots, it looks as if she’s having one of those “mommy moments” telling James, with clenched teeth (you know, when we’re trying to be discreet while reprimanding our kids in public) that he must be crazy if he thinks she’s going to carry his book bag all the way…