I think reclassing is common for athletes as @laxmom24 said. I also live outside Boston and my current 9th grade son will be going to a highly-ranked ISL school as a 10th grader. The two school schools he applied to only admit 5-10 rising 10th graders and the lacrosse coach at one asked if he would consider reclassing. We were a little surprised since he is an all-honors student with a 4.5 GPA, a class officer, scored 93rd percentile on the SSAT, etc. etc.
I think this may be suggested for current 9th graders in particular because even if the kid is a strong candidate, there are only 5 spots for 10th grade, but would be a slam-dunk showing you already get As in the same honors courses among the pool ~70 rising 9th graders.
Also, matriculation numbers matter. For better or worse, many of the top college numbers you see are for recruited athletes, not necessarily the academics. So, if a coach can get a current 9th grader who is still physically developing to reclass, it will give the coaches one extra year to help with the recruiting process of a child who may be bigger, stronger, and faster (and help the school win more titles, pay an extra year of tuition, raise the test score numbers, etc.).
The coach said even though he is a strong candidate, so are all the other rising 10th graders who have similar stats but could be reclasses, coaches for all the sports will be fighting to get their chosen kids in, and some kids may be possible recruits in multiple sports (not to mention the non-athlete applicants who may be gifted artists or generally strong academics). Getting one of those 5 spots will be far tougher than getting in with the 9th grade masses, so you can see where someone would reclass to get an edge.
My son was accepted at both. Aside from him, there is another boy from his lacrosse club and I overheard a parent last week saying her son was a reclass going into 10th, so at least 3 of the 5 incoming 10th graders are male athletes, one of which is a reclass.