"Playground duty" is a term used in the UK to refer to watching over the playground during break/recess/lunch.
In primary schools this is usually done by teaching assistants or "dinner ladies" (who probably have a name like "nutrition operatives" now), while the overworked teachers cram chocolate and caffeine down their necks and do marking or plan lessons. My tone is lighthearted, but I'm really sad about the conditions in the UK state education sector right now. Anyway, we loved the dinner ladies at primary school, and they loved us. I wonder if they would have loved us as much if they knew how much energy we put into stealing chocolate bars and sneaking into places we weren't supposed to be.
In secondary schools playground duty is done by teachers or prefects. I remember at my high school, if the Headmaster (see wen i woz a kid - I) happened to be watching the playground that day, nobody would even dare to kick a ball too hard. That guy was good at what he did. He had two code names so he could be semi-safely discussed, one was "Batman", because of the cloak he wore and his crime fighting abilities. The other was "God". I have vague memories of being on the prefect rota and sometimes having to do playground duty myself. Incredible.
There is no such thing as a perfect prison, and in every school I've ever attended, there was a vibrant subculture of finding the blindspots in the sightlines and exploiting them. Kind of makes you proud to be a human for a change.