* No Overtime: Many state high school associations do *not* have overtime in regular season games. The game ends in a tie, and both teams receive a half-win in the standings.
* Overtime: Some state associations *do* use overtime in the regular season. The specific overtime format (e.g., sudden death, multiple overtime periods) will vary by state association. This usually involves a series of possessions, starting with one team's possession from the opponent's 10-yard line, then alternating possessions. The first team to score more points wins.
To know definitively what happens in a particular state, you'd need to check the rules of that state's high school athletic association.