NFHS 3-5-ART. 10 . . . A time-out occurs when:

a. An apparently injured player is discovered by the official while the ball is dead and the clock is stopped and for whom the ready-for-play signal is delayed, or for whom the clock is stopped. The player shall be replaced for at least one down, unless the halftime or an overtime intermission occurs. This time-out, if not charged, is an official's time-out.
b. An apparently unconscious player is determined by the game officials. The player may not return to play in the game without written authorization from a physician (M.D./D.O.). This time-out, if not charged, is an official's time-out.

c. The official discovers a player who is apparently bleeding, or has an open wound, or has an excessive amount of blood on his uniform. Such player shall be considered an injured player as in Article 10a (See Communicable Disease Procedure).