Guys, the idea of them staying on campus is so they are safe during school hours. If they leave the grounds (especially in primary / elementary years) then they're quite vulnerable to attack. It is the school's obligation to parents that they're kids will get through the day and that should always be their main priority.
In all honesty, there is no reason why a student should ever leave the grounds. The school should provide enough resources on campus so they don't have ever have to leave. You've got a canteen, use it.
It's not a matter of the kids being 'old enough' to be out on their own, it is a matter of their safety being the school's top concern and that can't be provided if the students are wandering around at various times of the day.