This is like a Zeno's paradox of the catch rule. Although the catch rule might be different in the HS league then the NFL.
To rule it an INT, you have to rely on the rule that a receiver doesn't become a runner until the catch is complete.
So even if they catch the ball with control while going to the ground, even if the knee is down ending the play. Or touched by a defender while on the ground and possessing the ball with control. If the ball pops in the air before the catch is complete, the play isn't over.
Same weird paradox of a receiver catching the ball with control while crossing the plane of the goal line with two feet down and then losing control when hitting the ground. Not a TD even though they controlled the ball in hand and broke the plane.