why does Gene stand on the tree as if on a troopship?

The tree from which Phineas and Gene jump has been mastered by older boys; Gene and his friends, however, are a little younger in a class known as Upper Middler . Phineas and his pals are all young and not of draft age. Boys who had jumped out of the tree before are now all off fighting. Jumping off the tree hence becomes a symbol of war. These are the games of war that boys play to simulate involvement. The "Troopship" is their battle station, it is their launch into their own sort of war.