Airborne Receiver

[NWMichiganRef]

NFHS rules. In high school football, the rules state that the ground CAN cause an incomplete pass. In our game last week we had the situation where an airborne receiver (in this case it was the defender) go up and catch/gain possession while in the air. When he came down his knee hit the ground first (in the field of play) and then the rest of his body hit the ground, also in the field of play. The contact with the ground caused the ball to pop out. The linesmen ruled this a catch due to the knee hitting the ground first after possession was secured. It wasn't a very popular call but I think we called it correctly. Was this the correct call?