Fourth and 5 from A's 40. A2 rolls out and runs from B1. while scrambling A2 throws the ball away, clearly intentional grounding. WH throws flag and signals appropriately. He informs B's coach that he must decline the penalty to gain possession of the ball. This was a HS game. I was watching this game. ISn't this penalty a yardage and loss of down penalty, such as OPI? NFHS rules.

Yup - should be B's ball, 1st and 10 at the yard line 5 yards back from where the illegal pass was thrown.

I believe this was covered in a recent PI thread, but the key here is that loss of down means "loss of the right to repeat the down." If it's fourth down and you can't repeat the down, you best had reached the LTG during that down.

Canadian Ruling
 

Intentional grounding in Canada is a loss of down at the point of release. Declining the foul results in an incomplete pass, so a turnover on downs, B 1D/10 @ PLS (A's 40). Accepting the foul gives yards not gained at the point of release, which is of course behind the line of scrimmage. So, B 1D/10 @ point-of-release, which is better (by definition) than A's 40.

How far down field was the ball thrown? If the QB is far enough down field, A will want to decline the penalty and get the ball in better field position.

I imagine he was behind the LOS because they said Intetional Grounding, not illegal forward pass. :)