By refusing to administer a district-mandated test to their students, teachers at a Seattle high school have sparked an “anti-testing movement” that is picking up steam by the day.

The Chicago Teachers Union on Monday became the latest to jump aboard and throw support behind the Seattle boycott, which has shined a light on the growing resentment among teachers, labor groups and others toward standardized tests.

“I think it’s important for us to go on record about this because we are likely to start seeing a more active anti-testing movement in Chicago,” said Karen Lewis, president of the city's teachers union.

Standardized tests were the cornerstone of the 2001 No Child Left Behind law, but its greater emphasis on such results has prompted a backlash from teachers and unions in recent years. Education Secretary Arne Duncan also has criticized what he calls overreliance on test preparation.

While standardized testing in general has grown unpopular, teachers at Seattle's Garfield High School are specifically protesting the Measures of Academic Progress exam and for the past three weeks have refused to administer it to their students.

The teachers are rebuffing direct orders from the city schools Superintendent Jose Banda to administer the test and ignoring threats of 10-day unpaid suspensions. They argue that the test is fundamentally flawed and takes away from valuable class time.

“They are frustrated that the test doesn’t line up with the curriculum, doesn’t provide feedback they can use to teach their students and ties up the computer labs and libraries for students who are not taking the test,” said Jonathan Knapp, president of the Seattle Education Association, explaining several of the specific complaints teachers have voiced about the Measures of Academic Progress.

Critics say the tests are a waste of time and are unfair to low-income and minority students.

Others argue that the teachers’ main fear is that students’ performance will reflect poorly on them.

The Measures of Academic Progress is a computerized adaptive assessment of math, reading and other skills. Results are used as a part of Seattle teacher evaluations, and many teachers and their unions have vehemently opposed linking student performance to teacher reviews.

The idea of tying evaluations to test scores also has divided labor groups and the White House, which has voiced support for the idea. Critics of the boycott suspect the teachers’ real motivation is to undermine attempts to connect their evaluations to their students’ test results.

“Ostensibly, [the Seattle teachers’] protest is about the overuse of tests, the instructional time that those tests devour, and the culture of soulless data-driven instruction that animates today’s brand of school reform,” said Michael J. Petrilli, executive vice president of the conservative Thomas B. Fordham Institute, an education think tank.

“The real reason the Garfield teachers attack the MAP, one must presume, is because it’s a small part of Seattle’s new teacher-evaluation system. … It can pinpoint precisely where students are on the achievement spectrum and can give teachers full credit for any progress.”

Last year, teachers at a Chicago high school boycotted a district-mandated test and helped eliminate it. Teachers in other states have expressed similar disdain for standardized assessments, but full-blown boycotts have been rare until recently.

Instructors now are being publicly encouraged — by union leaders and others — to stage boycotts, even if the tests are required by school district policy.

“I, along with 3 million educators across the country, proudly support our members’ efforts in saying ‘no’ to giving their students a flawed test that takes away from learning and is not aligned with the curriculum,” said Dennis Van Roekel, president of the politically powerful National Education Association, the nation's largest labor union.

Seattle Public Schools warned teachers last week that they may face an unpaid, 10-day suspension for refusing to administer the test, which must be given by Feb. 22.

Mr. Banda, Seattle’s superintendent, increasingly is being urged by local media and others to take control of the situation and stop instructors from blatantly disregarding district policy. He was appointed in May to take the reins of Seattle schools after four years leading schools in Anaheim, Calif.

Last week, he announced the formation of a task force to study the Measures of Academic Progress and similar assessments.

Even before the boycott began in early January, Seattle schools planned to re-evaluate the benefits of the test at the end of this school year.

It’s possible that the test would have been nixed without the boycott.

For now, however, Mr. Banda is pleading with his teachers to give the exam. At the same time, powerful labor leaders and others are lining up behind those same instructors and urging them to stand their ground.

“I am asking as your superintendent that teachers follow our policies and procedures and administer this assessment for our students,” Mr. Banda wrote in an open letter last week. “This is especially important for our students who are the most at-risk academically. I am hopeful we will continue to work together in support of our students.”