The Always War by Margaret Peterson Haddix
When Gideon Thrall gets chosen to receive a medal of courage, Tessa is overjoyed. Their small town now has something to celebrate about! But Gideon runs off the stage, saying her is not a hero and he doesn't deserve the medal. Tessa is very confused, so she follows him to see why he reacted in such a way.
Soon, the two teens are caught up in a mystery that is dangerous and frightening: why are they fighting this war? And why are the computers giving them contradicting information about the battle sights and enemy territories? Can they be brave enough to face the truth when no one else is?
Tessa's body reacted as quickly as her mind. Before she was even concious of moving, she was already on her feet and running toward Gideon. Her legs tangled in the blanket, but she kept going, diving for Gideon. She knocked him sideways onto the floor of the plane, so if any of the enemy were already trying to shoot him, the bullets would just whiz harmlessly past. But this wasn't enough. It wasn't enough to tackle him, to hold him down, the mud form her ragged clothes rubbing off on his spotless white uniforn. Guns could ne reloaded, re-aimed, fired again and again and again.
Tessa rose up and slammed her hand against the control on the wall.