False start? or offsetting?
 

NFHS rules. Offense is hustling to the line. Defense subs and player runs off late.
Offense false starts while defense has 12 men on field. Is this a false start on offense? Or do we have offsetting penalties?

A false start is a dead ball foul. Illegal substitution (in this case) is a live ball foul. The false start means that the play never began, so the illegal substitution never happened.