Originally Posted by The Roamin' Umpire

All that matters when deciding touchback or safety is the reason the ball crossed the goal line in the first place. A forced the ball into B's endzone, therefore this is a touchback. (Just as if the B player fell on the ball, or if he was tackled, or if he went to one knee on his own.)

The rule reference is NFHS 8-5-3c.