A review using current data finds that Wisconsin does perform better on test scores than the non-union states, but not as dramatically as suggested in the Facebook post. And there is at best limited evidence that unionization played a causal role in shaping differences in test scores.
Hans Bader builds off this analysis:
In 2009, Virginia ranked in the middle of states on the ACT and SAT, and in 2010, it actually outranked Wisconsin on the ACT (12th vs. 17th in "average composite score"). The reason it doesn’t rank higher on the SAT is because so many of its students take the test including marginal students who wouldn’t even take them in another state. (Wisconsin boasts a higher average SAT score than Virginia partly because only "four percent" of Wisconsin students took the SAT, compared to "67 percent" in Virginia. Virginia’s lower average SAT score is a function of a larger pool, not dumb students or bad schools, as PolitiFact pointed out in debunking the false claim that Virginia ranks 44th .)