2000 kids at our Secondary school. At each lesson transition they all move rooms. In many schools there can be more students.
All staff supervise at doorways, along walkways, outsude toilets and at key pinch points.
One of their tasks is reminding students to put their mask on, or put it on properly. They hand out masks to those without. The whole system is extremely slick, efficient and mostly trouble-free, allowing lessons to start on time and students to feel secure in the corridors.
There is no way every member of staff is going to be able to remember and recognise every student exempt from wearing a mask, especially at top speed as the wave passes their station. Some will, but many won't, so I would say previous estimates of being asked where their mask was six times a day is likely to be an underestimate. They need a clear signal of exemption. If not a lanyard or badge, then to hold up a card or something.
It isn't only for the staff and the pupil themselves, however. Other pupils need to know which rules apply to which students. They are very accepting of different rules applied in different ways, and know generally about their friends and classmates, but not others from different yeargroups, at least initially.