I think the vast majority of reponses you've had have been less than supportive, because of the way you phrased your quetion / title, and then your OP.
You have stated, as we've gone through the thread what you actually want, but a lot of posters won't read 5 pages of posts.
You are right, it would be good if the system can be reviewed, as your ds did manage to get past the teacher and head off to the exit. Therefore it would be good to review the system.
All the posters saying that 2 full years plus 3 months in to school, even at 6, your ds ought to know if he can't see you / dh / whoever is collecting, then he goes back to the teacher.
In the same way that that failed, it seems what seems like a perfectly good system for releasing children also failed. Same as, of course the person collecting ought to be there, it happened (as these things do, once in a blue moon) your dh wasn't there.
I think you will get a far better response from the school if you ask to speak to someone calmly, in an 'acknowledging all the above points' way, and ask if the system might be reviewed, and that all the children might be regularly reminded what to do if they can't see their 'collector', than if you start off with "What is the legal responsibility of the school" way, which starts everybody on the defensive.