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

In the the novel, extraordinary children are required to contribute to their society in an adult way. Their skills need to be used: they aren't simply relegated to playing games or going to school. In fact, it is vitally important that their talents be put to work.
However, the children—as is shown in the novel—are also treated in an age-appropriate way, which is somewhat of a contradiction. They are told they are playing a game, even though they are in fact deeply entrenched in a life-and-death situation that depends on their abilities. This "sheltering" allows them to be children and alleviates them of responsibilities that they are too young to bear—which might be paralyzing.
The novel therefore takes a Romantic approach to childhood, showing that the children have a combination of skills, clarity, and innocence that the adults rely on to survive: the children truly are, in Wordsworth's words, "the father of the man."

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.