In the fictional Minnesota town of Lake Wobegon, made famous by public radio hostGarrison Keillor,"all the women are strong, all the men are good looking, and all the children are above average." But real life doesn't work that way — a fact that seems to have upset thousands of Baltimore City teachers this year after they got performance evaluations indicating that many of them could use some improvement.

This shouldn't come as a surprise. Improving the effectiveness of — and rewards for — good teaching has been a cornerstone of city schools CEO Andrés Alonso's reform agenda since he arrived in Baltimore in 2007. However disconcerting the slew of critical evaluations is for those accustomed to years of cursory, pro forma reviews, they're a sign the school system is finally treating the process with the seriousness it deserves.

For years, teachers complained about the arbitrariness of a system that gave some teachers bad performance reviews simply because a principal or supervisor didn't like them, while others received stellar marks due to favoritism. Clearly, a system based on more objective criteria was in teachers' own interest, especially at a time when salary increases and promotions increasingly depend on demonstrating effectiveness in the classroom. Teachers should be the first to demand greater transparency and objectivity in the evaluation process.

For the last two years, the city and state have been working toward providing exactly that. Instead of basing evaluations largely on personal observations, staff interviews and classroom visits by principals and administrators, the new system judges teachers' effectiveness by giving significant weight to how much their students' academic performance improves during the year. Growth in student achievement isn't some fuzzy abstract concept or subjective impression. It is real, and it can be quantified in test score results.

Replacing subjective evaluations with a system that measures how well teachers succeed in boosting student achievement is a much fairer way of evaluating teacher effectiveness and incidentally ought to go a long way toward eliminating the personal biases teachers complain about.

But it's also designed to spot weaknesses in a teacher's performance that need to be remedied. That helps the individual teacher and the school become more effective overall. It's no shame to be told that one can get better at what one does. The fact that more than half the teachers in some schools were given "performance improvement plans" after their evaluations doesn't necessarily mean the school system intends to fire them; it's simply an acknowledgment that no one is perfect. If we are to improve students' performance, it stands to reason that we will also need to improve the skills of those who teach them.

The new evaluation procedures were a key component of Maryland's successful application to the Obama administration's Race to the Top competition, which resulted in the state being awarded an additional $250 million in federal education funding in 2010. And though the new model was adopted over the objections of some teachers groups, it has solid support from federal education officials, the state board of education and lawmakers in Annapolis. The state board is making it mandatory for all Maryland school districts, to be gradually phased in over the next year.

But of course, once educators start using the new evaluation tools, not everybody can be above average, much less perfect. And the new measures inevitably will identity a small minority of teachers who truly don't belong in the classroom. A more transparent and objective evaluation system will give school officials the means to gradually weed such people out.

What we hope to see, however, is a system that develops into a more reliable identifier of good teaching practices that can be adopted systemwide to improve the quality of classroom instruction and produce even more rapid growth in achievement among city schoolchildren. If making sure that every city classroom has a highly effective teacher at its head is the ultimate goal of Baltimore's school reform effort, there's still plenty of room for improvement.