Don't forget the clock is stopped for penalty kicks also. There is no such thing as "stoppage time" in high school soccer. You stop the clock for the above reasons and when time is out, the game is over, no advantage, no waiting to see if the attacking team scores, no allowing a corner, etc..
If you're lucky enough to get to state, the game clock is official. When it buzzes at zero, game over, no matter what else is going on.