Keep in mind that the "system" existed before there were effective teacher unions, and teacher strikes didn't become widespread until the 1970s and 1980s. Also keep in mind that five states like Virginia, where I teach, have no collective bargaining, effectively banning teacher unions.
Here are the 2007 ACT/SAT score rankings of those five "non-union" states:
North Carolina (47th); Texas (45th); South Carolina (39th); Georgia (26th); Virginia (25th).
This means, then, that the 24 states ahead of Virginia in SAT/ACT scores have teacher unions.
Just some food for thought.