If a high school team playing with nine players reaches a situation where one of the players must leave the game, the team is allowed by the rules to continue with eight players. If the player who had to exit the game was a runner, the last previous batter before him who is not on base is allowed to take his place as a courtesy runner.
Under this rule, a coach is allowed to continue a game with eight lineup batters, even if he has substitutes available. Any time that a vacant batting slot comes due to bat, the plate umpire is to rule an out.
If a player apparently had to leave the game for good, leaving his team with eight players, but is later able to play, he can come back into the game. However, depending on what happened, the player may need to have a doctor’s note that OK’s him to come back into the game.
[OBR: a team unable or unwilling to field nine players has to forfeit.]