where do you mark the penalty?

[TerpZebra]

NFHS rules.

1st and 10 for Team A at their 20 yard line

A1 receives the snap and drops back to pass.

B1 holds A2 downfield at the 35 yard line.

A1 then runs the ball past the line of scrimmage to his own 25 yard line where he throws a backwards pass which is intercepted by B2 at the 23 and run back for a TD.


My thoughts: it is a running play because the backwards pass came after A1 crossed the line of scrimmage. B's TD will not count because they obtained the ball with dirty hands. The penalty should be enforced where the run ended (A's 25 yard line). A's ball, first and 10 at the 35 yard line. However, how do we know where the run ends? We do not bean bag a backward pass, etc. We have no reference to mark the penalty.

Your thoughts? And please include rule references, if available.