Not sure I should bother to explain this again, but what the heck :) ... we did not previously escort children to the door of the classroom, only to one of 5 exterior gates/doors. Not even that far, really, used to stand 10'-20' for further away and just watch until they went in (past a staff member). Plenty of children (even some in KS1) have no adult escort or dropped at the gate. So congestion not an obvious issue. Things feel much more crowded now.
Lining up to go in was adopted for the expressly stated purpose of getting teachers to spend more time talking to parents in the morning, not less. The pupils were more independent before than they are now. We now have more not less cotton wool.
I'm increasingly suspicious that the HT (relatively new, yes!) just didn't like the "look" of what we had been doing for years, so came up with this purely to be conventional, not because it actually works well in our setting.
(Sigh).
I can't wait to see what happens when it next pisses down. There'll be mutiny among the staff if they're expected to stand in the rain or very cold.
Staff supervision: I don't think there is any until the teachers come out & children queue up, about 5 min. before official start time, and more than 5 min. before they go in.
We have also had directives (widely ignored I think) that pupils & parents must not enter school yard more than ten minutes before school-start or home-time. Like most British schools, the waiting school yard is well fenced off from any access to pupils & classrooms.