Best I can come up with is that Luke proved in the simulator that he was among the best they had at precision bombing and, like you said, that gave him priority in the third echelon to attack the exhaust port. Wedge and Biggs were assigned not as underlings, but as bodyguards. Luke didn't have de jure command authority over them, but his status as the designated shooter gave him de facto authority. We can see Biggs and Wedge questioning his orders ("Are you sure we can pull out in time?" "Are you sure the computer can hit it?" "What about that tower?"), which is consistent with my theory. They're trying to keep the whiz kid alive long enough to hit the exhaust port, and it's painfully clear to them that maybe the kid can hit the target, but he knows almost nothing about staying alive in a starfighter.
Best I can come up with is that Luke proved in the simulator that he was among the best they had at precision bombing and, like you said, that gave him priority in the third echelon to attack the exhaust port. Wedge and Biggs were assigned not as underlings, but as bodyguards. Luke didn't have de jure command authority over them, but his status as the designated shooter gave him de facto authority. We can see Biggs and Wedge questioning his orders ("Are you sure we can pull out in time?" "Are you sure the computer can hit it?" "What about that tower?"), which is consistent with my theory. They're trying to keep the whiz kid alive long enough to hit the exhaust port, and it's painfully clear to them that maybe the kid can hit the target, but he knows almost nothing about staying alive in a starfighter.