Arlene C. Ackerman is superintendent of the School District of Philadelphia

Teachers are the bedrock of our schools and the single most important key to student success. To achieve great results, every student needs a great teacher, and every teacher deserves a fair and accurate evaluation that enhances their capacity to grow and improve without fear that the process will threaten their position or their professional standing.

To put the best interests of our children front and center, the School District of Philadelphia is determined to do everything in its power to recruit the best, brightest, and most dedicated teachers; to encourage, reward, and retain our highest performers; to provide meaningful assistance and support for teachers who are struggling to be successful and effective; and to create a comprehensive system that provides all instructional staff with ongoing opportunities for career and talent development.

We stand with President Obama and U.S. Education Secretary Arne Duncan in placing an aggressive and unrelenting focus on teacher effectiveness as a critical factor in creating better public schools. If we are committed to student success, then it is up to all of us - teachers, administrators, parents, policymakers, and legislators - to make a commitment that all of our teachers will have the skills they need to be successful educators and that all will be equitably placed where their talents are most needed.

We are morally obligated and collectively responsible to ensure that anyone entrusted with the education of our children is capable of doing a great job, is recognized for the excellence of their performance, and is justly rewarded for results. If we care about the success of our students, we must also care about the success of their teachers and treat them as the professionals they are.

Recently, the New Teacher Project released a report on "the nation's failure to assess teacher effectiveness, treating teachers as interchangeable parts." The two-year study describes a "widget effect" that has prevented schools and school districts from "recognizing excellence, providing support, or removing ineffective teachers."

The study, available at www.widgeteffect.org, describes a "national failure to acknowledge and act on differences in teacher effectiveness" and faults teacher-evaluation systems that codify the "widget effect" by allowing excellence to go unrecognized and the need for improvement to go unaddressed. The authors noted that less than 1 percent of 40,000 teachers in the study were ever rated unsatisfactory.

The Philadelphia story is no different. Out of a teaching force of more than 10,000 in the district, only 13 received unsatisfactory ratings, and only five were removed from the classroom. Because struggling teachers who are performing below an acceptable level of effectiveness were not identified, they could not be appropriately assisted and supported and given an opportunity to improve.

We cannot hope to close the opportunity and achievement gap that exists in our school district, help students from all backgrounds achieve at high standards, and realize the goals of our district's Imagine 2014 strategic plan without putting great teachers in the places where students need them the most. To meet the needs of children and schools fairly, our district needs greater flexibility to assign staff to schools that best match the talents of teachers with regard to subject, site, and area needs.

Some Philadelphia public school students do have access to great teachers, effective principals, and excellent programs. However, "some but not all" is not an acceptable standard. All of Philadelphia's children deserve a chance to dream and succeed, and all need skillful classroom teachers able to provide them with love and limits as they strive to learn.

As we end this school year and prepare for the next, let us commit that our school district staff, parents, and community leaders will work together to guarantee that neither students nor teachers will suffer from a "widget effect" in Philadelphia's public schools.

E-mail Arlene C. Ackerman at superintendent@philasd.org.