Baltimore City schools CEO Andres Alonso may have been waxing enthusiastic when he said this week that current contract negotiations with the city teachers union could result in a "remarkable re-conception of how teachers get paid." Whether such a sweeping revolution is feasible in the few days left until the end of August, when Mr. Alonso expects to sign an agreement, is still up in the air. But if things do end up going his way, Baltimore could become one of the first school districts in Maryland indeed, in the nation to formally tie teacher evaluations to student achievement.

Mr. Alonso has long argued that teachers and principals should be held accountable for the performance of their students. He’s made putting a good teacher in every classroom a top priority, and he’s closed dozens of failing schools and replaced them with better alternatives. But the idea of rewarding teachers on the basis of how well their students perform academically got a big boost this year when the Maryland General Assembly passed a law requiring that up to 50 percent of a teacher’s evaluation be based on student achievement.

The aim of that legislation was to put Maryland in a more competitive position to win additional federal funds through the Obama administration’s Race to the Top contest, which was set up to encourage reform efforts in states that demonstrate a serious commitment to change. Maryland is currently a finalist in the second round of the contest, after Tennessee and Delaware emerged as the only winners of the first round earlier this year.

Although some Maryland school districts including Montgomery County, the state’s largest have resisted modifying the way teachers are evaluated, Baltimore City embraced the concept because it could bring millions of new dollars to its cash-starved school system. City teachers union president Marietta English said that while there were still many details to work out in the current contract talks, the union would do nothing to jeopardize the state’s shot at winning an RTTP grant.

Of course, the devil is in the details, and in the case of tying teacher evaluations and pay to student achievement, there are many questions still to be resolved and no clear path to success because so few states have any sort of track record at all about how to make it work.

For example, while it’s relatively easy to evaluate math and reading teachers based on the test scores of their students, most teachers don’t actually teach subjects covered by state or national standardized exams: music teachers, gym instructors, first-grade teachers and many others lead classes for which no formal, standardized exams exist. How are they to be judged under the state’s rule that student achievement must count for up to 50 percent of their evaluations?

Nor has the state determined exactly how much progress students should be expected to make in any given year in order for a teacher to be graded effective or ineffective. And how should one judge the system’s overall effectiveness in a city like Baltimore, where more than 6,000 students are truant on any given day? The achievement gap between regular attendees and habitual truants can be as much as 20 percent, but should a teacher be blamed for the poor performance of students who don’t even show up for half the year?

In Baltimore, many kids make great progress during the year in reading or math, yet still fall short of target goals set by the No Child Left Behind law because they’ve missed so many classes in previous years or because the instruction they did get was inadequate. Should the fourth-grade math teacher whose students do poorly be blamed because the third-grade teacher who had them the previous year didn’t teach them how to do fractions?

Mr. Alonso was clearly aware of these difficulties last month when he seemed to play down the importance of standardized test scores and suggested that growth in student achievement was a more reliable indicator of progress. But how do you measure growth if not by testing?

Squaring that idea with the state’s requirement to base a substantial portion of teachers’ evaluations on student performance will require some truly innovative approaches. But Mr. Alonso has shown an uncanny talent for working with the teachers union to come up with solutions to such problems, unlike Washington’s embattled schools chief Michelle Rhee, whose combative style has thrust her stewardship of the schools into the midst of a hotly contested mayoral primary contest there.

But even given the Baltimore teachers union’s relatively good relations with Mr. Alonso, he surely will need all diplomacy and persuasiveness he can muster to bring this year’s contract negotiations to a successful conclusion.

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