"Other children manage"
At the school gate on Thursday there are children with their kits. Sam's friend has his. The girl from number twelve is always ready, always tidy, always on time. Other children manage, and yours doesn't, so the difference is yours.
Two things. The first is what you can see of other children. You see them at the gate, dressed, with kit, for ninety seconds. You don't see the morning behind it. The friend's mother packed the bag last night and put it by the door. The girl from number twelve has a list on the fridge and a father who stands at the bottom of the stairs. The children who manage are, as far as you can tell from the gate, children in houses with arrangements, and the arrangements are invisible to you for the same reason your own parents' were invisible to you, which is that a working arrangement looks like a child who just does it.
The second is the range. Gathercole's working memory line has a spread around it at every age. Some nine-year-olds hold more than others. Some are early on Munakata's transitions and some are late, and all of them are normal. The friend who remembers his kit may be at the top of the spread for nine, and Sam in the middle, and both of them will be at the top of the spread for fifteen in six years, because that's how development works. Comparing your child to the most organised child at the gate is comparing him to the top of a range and calling the distance a fault.
There's a version of this that's worth taking seriously, and it's chapter 22's. If your child is far below the range, not at the middle or the lower end but well outside it, and the school sees it too, that's a conversation to have with the school and the GP. But "other children manage" is nearly always a comparison with the visible end of a normal range, and the invisible end is at the same gate, going back for its kit.
And notice what you're doing to your own child by comparing. He's at the gate too, and he can see the friend with the kit, and if the comparison is said aloud he learns that he's the one who doesn't manage, which is the label with a witness. The friend's arrangement is worth learning from. The friend isn't worth measuring against, and the child can tell the difference between the two even when the parent can't.
Ask the friend's mother, if you know her well enough, how the kit gets into the bag. In my experience the answer is a laugh and a description of an arrangement, and the arrangement is what you're about to build.