In my experience, you knocked and went in, a brief discussion, “the event” and out.
The Head’s door was not locked but he had traffic lights.
Green = knock and enter
Orange = Do not knock or enter
Red = Not in
At my other school, I did see a boy of 12-13 having to make his own preparations for a caning by fetching a desk from the classroom across the corridor to take into the Deputy Head’s office as something to bend over whilst having the cane. He was in tears, I don’t blame him as this whole process added to punishment.