A situation from a youth league game: A runner was on 2nd, and the ball was hit to the outfield. Runner touched third and ran home. There was no throw home, but the runner didn't touch the plate. His momentum carried him past the out of play line; after teammates and coaches told him that he missed the plate, he went back and touched home without being tagged and before an appeal could be made. However, the umpire called him out for leaving the field of play. Was the umpire correct?

The league plays under Cal Ripken rules, but I don't have access to their rulebook, and have been unable to find the rules online (other than ordering a copy of the rulebook).