As a parent of a 15 year old and a 13 year old and a 10 year old and a 8 year old, 2 year old, 3 1/2 month old, I can tell you it gets old Caroline, when they keep forgetting their stuff at school or where ever.
It gets to the point where they have to suffer the consequences for their forgetfulness in order to not keep making the same mistakes, to not keep forgetting things all over the place because when they become adults in the real world that just isnt going to cut it.
When you get out the vehicle, you have to check to see if you have everything. I have to check to see if I have everything when I go inside, my purse, shopping bags, whatever. That is my responsibility, no one elses. The last thing I'm doing is making sure a 15 year old brought in his books.
I can also let you know this, if any of my kids E-ver said to me that I am the one who made the mistake (and that is something they would say too) because I did not check to see if they forgot something they were supposed to not be forgetting, they would be punished for that comment. I wouldnt punish them for forgetting their stuff in my car, their having to face the natural result of their action would be punishment enough but they would be punished for daring to speak to me that way and showing such disrespect.