Although the Washington Teachers Union may be filing a class action lawsuit against DCPS for firing 241 teachers (two-thirds of whom were rated ineffective), DC's new IMPACT evaluation system is not rating more teachers as ineffective than what the standard distribution of teacher performance would suggest.

Most economists say the quality of the current teacher workforce looks something like a bell curve: approximately 15 percent of teachers are highly effective, 15 percent are ineffective, but most are somewhere in the middle.

In Washington, DC this past school year, 16 percent of teachers were rated highly effective, 62 percent were effective, 18 percent were minimally effective and 4 percent were rated ineffective.

In most districts, 99 percent of teachers get a passing mark on their evaluations, even when students are chronically underperforming. So while DC's IMPACT gives a far more accurate assessment than what nearly any other American school district has been able to provide, it is still likely underestimating the number of underperforming teachers in the system. Of course, it remains to be seen what will happen next year to the 18 percent of teachers with a minimally effective evaluation rating. Will they improve? Or with another year of student performance data, will they be rated ineffective?

IMPACT may have some kinks to work out, but any system that at least distinguishes among the good, the bad and the ugly gets our vote.