I'm unabashedly stealing this question from the NCAA forum because, while the NCAA has some different rules, the play has significance at the HS level - especially since many teams are going to the "no huddle" offense.
"Team A breaks the huddle with 10 players, who assume position in the formation and become set. A35, who did not participate in the previous play, realizes he should be lined up as a back on this play, and enters the field from the team area. After he is inside the 9-yard marks, but while he his still running to his position, the ball is snapped. His movement is otherwise legal (not forward, no other players moving while he is running onto the field.) Ruling?"
How would you handle this in your High School game?