But the unexpected happens, all in slow motion. I step back to allow an 11-year-old, carrying his dinner, to pass in front of me. At the same moment a primary 1 child, impatient to pass unseen, dodges into the tight space between me and the wall. He is only the height of my knees and I fall over him, hitting the wall before I crumple to the ground on top of him.
My uncontrolled left foot trips the primary 7 in front who crashes to the floor, face down in his custard.
My first concern was a selfish one, for my dignity. Half our pupils were nearby but, to their credit, their reaction was one of concern. Nobody was hurt but later, when I reflected on the incident, I came to the conclusion that the primary 1 pupil may have been the victim of a form of blatant discrimination which exists, inevitably, in schools.
Campaigns against racism or sectarianism are commonplace. BQ and Asda have enhanced their reputations by combating ageism. But schools, particularly primaries, still cover up a discrimination which receives no publicity. I refer to the curse of heightism.
The child's lack of centimetres led to my bulk crashing on to him. Had I stumbled against an adult, I wouldn't have fallen over. The child running around the corner of the playground is likely to crash into an older one, then hit the ground, heightism again.
The classroom contains pitfalls for all ages of children. In the interests of good learning a teacher should spend much of her time on her knees. Only then can she appreciate a child's classroom obstacles. Blackboard writing may look fine to an adult but may be obscured by a slash of daylight on its too-shiny surface when viewed from child height.
Only when she is on her knees can the teacher spot when a child's sight-line to the blackboard is blocked by a taller classmate or that her displays are worthless because they are too high. Highest of all usually is a map of the world which, complete with minute printing heightism, handicaps geography.
Heightism is not always bad. Sometimes it works in a teacher's favour. In a crowd of children, the teacher has an advantage in spotting misbehaviour and can be more relaxed about it too.
At child level, a group of bodies of similar height make a dense forest and individuals can't see farther than those closest to them. Now, I remind myself to make less fuss about some misbehaviour since I am only drawing attention to it. I have the best view and often the other children see nothing.
But that's enough sympathy for children. Heightism catches out adults too.
Low chairs are vicious and to be avoided at all costs unless you wish a bad back for days. I prefer to kneel to be at child height but it now causes me knee trouble and, occasionally, problems in standing up again. Who wants some cocky 10 year-old to grasp your elbow, calling out in a deliberately loud voice: "Hold on to me, sir. I'll help you up."
In the cold light of a new day, I saw that, in my lunchtime incident, I was the victim, not my primary 1 friend. While I floundered on the floor, sticky with custard, he slipped from beneath me and skipped off to the playground, unhurt and without a backwards glance. So much for sympathy.
If you can sue for a trip on a stray dining-room chip, do I have a case when I trip on a primary 1?
Brian Toner is headteacher of St John's primary in Perth.