Virtually no info out there in a consolidated way, but you can look at individual school websites which sometimes post scores. Imperfect, but reasonable, to track ISL high school standings, and assume those programs with middle school feeders will roughly align, at least with tiers (ISL AA, ISL A, top teams, etc.)
Some rules of thumb:
--Typically, Stone Ridge is very strong at the middle school level, unsurprisingly. SSAS is perennially in the same category.
--Potomac and Holy Child are typically upper middle tier.
--High academics downtown (Sidwell, NCS, GDS) are not as strong normally.
--Holton-Arms, Flint Hill, Bullis, and others usually wind up in between.
--Smaller Virginia independents are more lower middle tier.
Two or three top club players can swing a team to winning records for a year or two for the middle-tier teams.