...because this is going to continue for quite some time. DD will be 10 in July, and I swear -- she would indeed forget her head if it wasn't attached!!
And her classmates and friends are the same. It is the rare kid who keeps track of his/her stuff responsibly -- in my experience, anyway.
I've seen one girl put down her lunch box in the hallway after school to load her backpack, then walk off with the lunch box sitting in the middle of the hallway.
DD came home from a sleepover without her sleeping bag (one would think you'd remember at LEAST that, right?).
And on and on.
I'd put labels in everything, and don't send what you don't mind losing.
I know how you feel, believe me, but I've found most kids are like this -- boys and girls alike. And some of the kids who do it at DD's school are among the brightest (advanced math, etc., so presumably they have pretty great mental capacity).