I think a top high school team would certainly be competitive if not outright beat the WNBA all-stars. that's just based purely on the physical differences. now if that high school team doesn't have any real shooters, it's possible the WNBA could win if the high school bigs have no real post game.
just drawing from my own experience in high school playing against a female friend of mine in gym class who was a top all-state athlete (including basketball) where I was a decent player (not on the school team but was more than capable of holding my own against guys on the team who were also in my gym class --> in full disclosure, my school's team really sucked) and would easily outplay her on the court.