This is the official NFHS
interpretation
SITUATION 1: The opponent makes contact with the free-throw shooter before the free throw reaches the basket. The free throw is missed. RULING: The official should rule a violation on the opponent and a personal foul. (9-1-2g Penalty 2b)
SITUATION 2: After A1 releases the ball on a free throw try, B1 steps into the lane and backs across the free- throw line to box out the free-throw shooter then makes contact with the free-throw shooter. The free throw is missed. RULING: The official should rule a delayed violation on the opponent. A1 will be awarded a substitute free throw and the contact is ruled a foul. The substitute free throw would be administered with the free-throw lane spaces unoccupied. (9-1-2g Penalty 2b)