As I remember, the main characters played strategy games at playtime, rather than ball games with the rest of the boys, making them outcasts. But one playtime, aliens land in the playground and challenge the children to a battle for the Earth.
There is some kind of playground equipment that becomes the children’s fortress that they defend. The main characters organise the rest of the children to fight off the invasion. The aliens were highly militaristic and I think insect-like.