All good points, Fire.

I used the playground example for several reasons:
- teachers on duty deal with all students in their zone, most of whom are not 'their' students. Consequently there is an increased risk of making a controversial call because the teacher may be unaware of certain info regarding the student or his parents' response pattern.
- the playground is where much of the bullying & intimidation & exclusion can take place. There's often more happening than meets the eye. Things can escalate very quickly with certain kids.

I mentioned the number of hours because as a 'behavioural' teacher I spent far more time on the yard than most teachers. Our students went out for recess with the regular-stream kids and I learned in my first week of teaching that it was much easier for me (and better for them) to go on the yard with them than it was to take my break. Over 34 years, an hour a day, it's at least 6,000 hours. 
That's a lot of time dealing with other people's children.