from ncaa perspective, if A is subbing then C stands over the ball while R gives iron cross. Once R is satisfied that B has matched or chooses not to, the R will waive off the C.
BUT, if A is not subbing, then there is no iron cross signal & C steps away from the ball once he sets it.
All this said, if NFHS has no subbing rule for B to match up, then I'm confused why U needs to stay on the ball.