I am not a football official, but officiate other sports and routinely read all forums here.

I have a question about fake field goal and extra point tries under NFHS rules.

1. Ball is snapped accurately to holder whose knee is on the ground as he catches the ball. Can he now legally get up and advance the ball or must his knee be off the ground before receiving the ball from the center.

2. Does this change on a muffed snap; and if the snap is short hopped to the holder can he still get up and advance the ball.

Thanks

As long as there is a player in position to kick, he may receive the snap while on his knees. That is an exception to what normally would be a dead ball at that spot. However, the holder must get off the ground if he plans to now run, pass or for that matter kick it.

A short hop recovery does not change anything, same restrictions as in 1.

A muff is different story. If the holder muff the snap and gets up to recover it, he may not return to the ground else the play is over, ball dead at that point.

You didn't ask about a high snap that pulls the holder off the ground. In that case, the holder may return to the ground so it can be kicked. He also can get off the ground and pass, run or kick it just as in 1.

A snap is considered a backwards pass, therefore it is a live ball, whether it hit the ground or not.

The holder can 'stand up' to retrieve the ball, but must return immediately to the ground to set-up the kick.

Canadian Ruling
 

A holder may catch the pass with a knee on the ground and from there, may hold the ball for a kicker (who is not required to be in any regular kicking form), run with the ball, or lift both knees from the ground and throw the ball in a passing play.