With reference to Ender's Game, how should high ability children contribute to their community?

An interesting question.

The short answer is, high ability children should contribute according to their ability, but also according to the need of the larger community.

(This isn't specific to children in Card's world, by the way: this applies to adults as well, like Ender's tutor Mazer Rackham, and to the way the society in the novel makes decisions about which families get to have third children, despite the population pressure.)

To expand these points a bit, all the boys at Battle School (and the few girls) are there because they are gifted. They are given special opportunities and special training because of their gifts, which in this case blend the physical skills of battle with the conceptual skills of planning and the complex emotional skills of leadership and motivation.

However, while they love these activities, and live for them, all of their rewards, training, and support are given them because the human race needs them. This collective need ultimately trumps individual talent and desire. In the end, the survival of the species (in the war against the Buggers) determines what gifted children can and must do.