>As I already wrote, I think that being able to locate pupils at all times is a legitimate concern for schools, and parents.

Depends on the age, really. It goes from all the time for really young children, to "locate in a reasonable amount of time, if needed". How long that is depends on the comfort zone of the parents of course, but has to account for allowing children to develop, learn by themselves and grow, or else it's wrongful imprisonment to be honest.

By the way, the "being able to locate all the time" wasn't even the goal of the school. It was to take attendance.