If nobody ever got off the WL, they wouldn’t have one!
But… I would guess that because there are so few BS that a rising 8th grader could choose from that their yield predictions are pretty good for their entering 8th grade class. I would expect less moved for 8th grade than 9th.
All you can do is confirm that you want to be on the WL and that you will attend if they offer you a spot (if that’s true!)