Re: I don't understand
It's generally not kids playing it. It's adults, all kinds, all ages and often moving around in cars for raid battles that they have arranged on social media in a similar way to how football hooligans meet up to fight (though the Pokemon ones are often bot managed). Raids can spawn up to 9PM, which is well after dark at this time of year at northerly latitudes. Raid meets after dark with several cars and people milling around are sometimes mistaken for dogging events and visited by confused police patrols.
As I understand it, school premises don't get anything on their grounds, but Universities campus are full of stuff. Churches, monuments, blue plaques are also pretty popular.
Find out when there is a community day and visit a park between 10AM and 1PM and you will see that it is mostly an adult pastime.