I was reffing with a new partner, who administered a free throw with two girls in the wrong lane spaces. I missed it until the ball was in A1's hand to shoot. Simultaneously, A2 and B1 realize they are in the wrong spaces and switch with each other (while A1 holds the ball preparing to shoot). The basket was good. What's the ruling if a) the basket is good and b) the basket is missed? We looked at each other, and realizing the basket was made we shrugged it off, since it was our fault, but I would like to know what the correct call would be for future reference. Yes, this should have been prevented!

If both players change positions at the exact same time, you have a double violation, and the free throw is cancelled. If the offense initiates the switch, it is an immediate lane violation and cancel the throw. If the defense initiates the switch, ignore the succeeding violation by the offense. If the throw is successful, ignore the violation. If not, give an alternate free throw.

This is a new free throw violation rule for NFHS this year.

TriggerMN is correct.

I would add that if no further throws are to follow, then you go to the arrow and inbound at the spot nearest where the simultaneous violation occured.

That's how I understood the rule, but I guess just surprise at the situation caught me off guard. It was the first attempt, so it should have been cancelled as the switch was simultaneous, and we should have proceeded to the second attempt. Thanks!