OK, so a random child has to memorise the names and faces of around 60-120 members of staff including the ones they will never have any dealings with beyond passing in a corridor once a year (and then also psychically knowing the names and marital status of all supply, temps, governors, clerks, advisors, consultants, counsellors, coaches, mentors, TAs, groundskeepers - and repeating this feat with new staff on the first day of next term).
If they somehow fail to achieve that, how do the conversations go?
'Excuse me, you.', 'After you, um, you', 'Yes, um, Mr?', 'She said I could - her over there, that woman', 'Yes, Missus, no, Mr'.
Miss and Sir works fine, not just for kids but for staff, too, as it saves having your first name called across the kids because somebody doesn't know who you are or has had a blank when they can't think of your surname out of 79 others. And like kids, when you get clusters of ages, you're going to have who share first names - Lucy, James, Stephen, Charles, Rebecca/Becca/Becky, Vicky/Vicki/Victoria, etc.