If a high school football player is injured during a game,

Most likely, the injured player's parents were asked to sign a waiver to not hold the school/football program liable if the player was injured during a game or practice. High school coaches are required by most state laws to warn players that they can get hurt playing football. I believe they basicly have to read the warning sticker in the helmet word for word to the players. If these things weren't done, the player's parents may have some recourse, but most schools protect themselves upfront in case there is a serious injury.