NFHS Case book 9-1: Comment... reads in part, "a team forfeits its right to any space not occupied
before the FT starts
(when the ball is placed at the disposal of the free thrower). Once a free throw starts, .... No player may enter OR LEAVE a marked lane space AFTER the ball is PLACED AT THE DISPOSAL of the free thrower until the restrictions have ended as outlined.
I would say it becomes your judgement, as the administrating official, as to whether you want to call this a violation. IMO, it is based on the level of play you are working.
Call it once and it should reduce the possibility of it occuring again (in that game)
.