Placement of ball?
 

3rd down on B's 10yard line. A5 rolls out and throws the ball away at the 15 yard line with no receivers in the are. Where is the penalty enforced from?
High chool rules.

Illegal passes are enforced at the spot of the foul. So this is a loose ball play and the foul took place behind the previous spot, the enforcement spot is the 15. And this is a loss of down penalty so 4th and (whatever) from the 20.

Peace

Illegal forward passes (in this case intentional grounding) are spot fouls. so if he threw it from the 15 (5 yards behind the LOS), you would enforce it from there. 5 yards and loss of down. 4th down from B's 20

JR, just as a clarification: the previous spot does not come into play on an illegal forward pass since it is a spot foul enforcement. Example: Pass thrown beyond LOS is enforced from spot of pass even if forward of the previous spot on what we might consider a loose ball play.
Correct me if I am wrong, always learning :)

7.5.2 Situation A

Penalize from the spot of the foul (where the run ended)

10-3-1 An illegal pass is not included in a loose ball play.

If it really matters to you, it is not a loose ball play, but it is enforced just like a loose ball play for IG. There was a time that the penalty for IG did not fit into an illegal forward pass definition and there was a debate over if IG actually was an IFP. Now the definition fits, but the penalty is enforced as if it was any loose ball penalty as to have IG you must be passing behind the previous spot (or LOS) in order to consider the other classifications of that penalty. Once you cross the previous spot then it does not matter who you are throwing it to. Either way it would be a foul enforced at the spot in either case. This is something I would expect the NF to ask on a test rather then what is actually relevant to the enforcement.

Peace

surehands, the easiest way to get this right is always remember that an illegal forward pass is a running play, not a loose ball play. Whether he throws it away from behind the LOS or runs 10 yards past the LOS, it's a running play and it's penalized from the spot of the foul.