The ball must break the plane of the end zone (doesn't have to completely cross) while a player is in possession of the ball and before he goes out of bounds or is tackled.
What they SHOULD do is hand the ball to the ref and jog off to the sideline like a classy player. Not like those NFL players that dance around like they've never scored a touchdown before in their...
One instance where that position would be able to score a touchdown would be in the case of a fumble, since centers are not allowed to have the football thrown to them, or if their is a trick play...