I know someone who transferred into Groton because they liked the smaller size of it compared to where they were at, but they had actually gotten in as a freshman in her first time around so that probably helped for sure. From what it seems sophomore year is much harder to transfer into than junior year regardless of whether or not you are already at a BS, and I doubt they would let someone repeat if they were already coming from another BS.
Another option I know a FEW kids did to get in after they got WL or rejected by their top choices was to go to a Junior BS –Eaglebrook, Rectory, Cardigan Mountain, etc– and then re-apply. Of course, this would only be an option if you were applying for 9th grade this year, but if you are, you could try to go there by repeating 8th or applying for 9th there, and then re-apply to you top schools for 10th grade after you finish up there. I am not sure how effective this really would be and I am not too familiar with the admissions process of JBS at this point in their cycle, but it might be worth a shot to explore this!