When Ender first gets to the battle school and is checking out his flash suit for the first time, the author makes it very clear that the gun with the suit does not work anywhere except inside the battleroom. Later on, when they put Enders army against the army which got to go in five minutes before them, and they were lined up along the inside wall of the battleroom, Ender made human shields to send his people out by flashing them in the hallway. Oops.