How many points are scored if the defensive team blocks the extra point and returns it for a touchdown?

In High School, the minute a try is kicked, it is dead, regardless if the kick succeeds, misses, or is blocked behind the line. If there is a fumble on a two point conversion and the defense takes possession, the ball is dead and the try has failed.

In the NCAA/NFL, if a PAT is blocked behind the line and possessed, or a two point conversion is turned over (interception or fumbled) and returned into the end zone it adds two points to the defense's score.