When the principal at P.S. 40 in Bedford-Stuyvesant, Brooklyn, talks about the impact on students of one of her best teachers, Malvola Lewis, her eyes fill with tears.

After growing up in homeless shelters, Lewis earned an education degree from Brooklyn College and returned to her old neighborhood to teach at P.S. 40, a historically hard-to-staff school. Now she’s one of the school’s strongest teachers; her students are making more progress than almost any other class in the school. And they love her.

Lewis is a terrific teacher. Despite her exceptional work, though, she (and thousands of teachers like her) may be laid off shortly because of antiquated seniority rules in New York City. The real losers will be children.

Teachers are professionals, and they deserve to be treated the way professionals in almost every other line of work are: evaluated based upon their work.

The city’s schools face a cut of $500 million in state funding for the upcoming year. Unfortunately, even with deep administrative reductions, we simply cannot absorb a cut that big without laying off teachers. Under New York state law, we’ll have to lay off teachers based entirely on how long they have been teaching, with no consideration given to their talent in the classroom or how well their students are doing.

Lewis, a third-year teacher, would be among the first to go of approximately 4,400 teachers who are vulnerable. Indeed, because of absurd seniority rules enshrined in our collective bargaining agreement, we have to fire junior teachers before we can lay off teachers who aren’t even teaching. Or before we can lay off teachers already rated unsatisfactory.

We’d also be forced to keep teachers in what’s called the “Absent Teacher Reserve” pool—a bureaucratic name for those let go from downsizing or closing schools but who remain on payroll. Many of these teachers haven’t applied for new jobs despite losing their positions as long as two years ago. And many who have looked for a job can’t find a school willing to hire them despite many vacancies. Yet none of these teachers can be laid off, even during a budget crisis.

Recently hired teachers are among our most passionate and creative, yet they make the least amount of money. Seniority-based layoffs therefore force us to discharge more teachers than layoffs based on merit. Fewer teachers mean class sizes could rise by as many as five students in some schools.

No neighborhood would be spared, but the hardest-hit area would be the Bronx, home to some of our most challenging schools and newest hires. Many of these schools have turned a corner and are now making incredible progress thanks to hard-working, recently hired educators.

In fact, a 2008 study in the Journal of Policy Analysis and Management found that the gap in teacher quality between our lowest- and highest-poverty schools has narrowed significantly during the last decade, largely due to the hiring of more successful newer teachers. Layoffs determined only by seniority would push some of these teachers out of a job.

And some schools would be hurt much worse than others. A newly released study by the Center for Reinventing Public Education at the University of Washington found that schools with high percentages of low-income and minority students are disproportionately harmed by seniority-based layoff rules as compared to schools with wealthier and fewer minority students. This is certainly true here in New York City; at schools with the highest percentage of black and Hispanic students, approximately 25 percent of teachers have less than two and a half years of teaching experience. Since we would be forced to get rid of these less-experienced teachers, schools with high-minority populations would disproportionately suffer disruptions to their learning environment.

Take P.S. 86 in the Bronx. The elementary school is 96 percent black and Hispanic, and would lose nearly 30 classroom teachers to layoffs based on seniority. By contrast, at P.S. 53 in Staten Island, a school that is less than 10 percent black and Hispanic, no teachers would be laid off.

Clearly, experience is important—but it’s not the only ingredient that makes a great teacher. In the unhappy event that we are forced to lay off teachers, we should institute a system that would allow us to keep the best teachers in our classrooms and minimize the harmful impact of layoffs on our students.

Each year, in accordance with collective bargaining provisions, principals evaluate their teachers based on classroom visits, lesson plans, and student work. The vast majority of our teachers receive “satisfactory” ratings, but, in our most recent ratings, about 1,600 were rated “unsatisfactory.” I would first get rid of those teachers.

Next, I would let go of the 1,100 teachers in our Absent Teacher Reserve pool who have been unable to find a job. Finally, I would let principals, who know teachers best, make decisions based on clear criteria: student progress, quality of teaching, and teacher attendance. Superintendents would review all of these decisions to make sure they were fair.

Teachers are professionals, and they deserve to be treated the way professionals in almost every other line of work are: evaluated based upon their work. Especially with our children’s future at stake.

Many teachers agree. In a survey of 9,000 teachers in two large urban districts, the nonprofit New Teacher Project found that nearly three out of four said factors other than seniority should be considered in layoff decisions. Even among teachers with 30 or more years of experience, a majority supported an approach based on talent, skill, and results.

Seniority-based layoffs are also opposed by many education advocates, including liberal groups like Children’s Defense Fund and the Center for American Progress.

When I think about my favorite teacher growing up, I think of someone who inspired me, put in extra effort to help me when I struggled, and challenged me to reach academic heights neither I nor my family knew were possible. I never considered—or cared—how many years she had been teaching.

Our students deserve to be taught by the most talented, dynamic, and effective teachers possible—whether they’ve been teaching for three years or 30 years. Children are not responsible for the budget mess facing our state, and they should not suffer because of adults’ mistakes.

We need to take dramatic action immediately to change these archaic and irrational rules and keep teachers like Malvola Lewis where they belong: in the classroom.

Joel Klein is chancellor of the New York City Department of Education.