Christopher Capozziello for The New York Times

Kyle James, legs tucked, and Vincent Walters in a sixth-grade social studies class at Cloonan Middle School in Stamford, Conn. The superintendent said the academic tracking system has been failing low-level students.

By the fall, most sixth graders in Stamford, Conn., will learn together in mixed-ability classes because, district officials say, the long-held system of academic tracking is unfair.

STAMFORD, Conn. — Sixth graders at Cloonan Middle School here are assigned numbers based on their previous year’s standardized test scores — zeros indicate the highest performers, ones the middle, twos the lowest — that determine their academic classes for the next three years.

But this longstanding system for tracking children by academic ability for more effective teaching evolved into an uncomfortable caste system in which students were largely segregated by race and socioeconomic background, both inside and outside classrooms. Black and Hispanic students, for example, make up 46 percent of this year’s sixth grade, but are 78 percent of the twos and 7 percent of the zeros.

So in an unusual experiment, Cloonan mixed up its sixth-grade science and social studies classes last month, combining zeros and ones with twos. These mixed-ability classes have reported fewer behavior problems and better grades for struggling students, but have also drawn complaints of boredom from some high-performing students who say they are not learning as much.

The results illustrate the challenge facing this 15,000-student district just outside New York City, which is among the last bastions of rigid educational tracking more than a decade after most school districts abandoned the practice. In the 1960s and early 1970s, Stamford sorted students into as many as 15 different levels; the current system of three to five levels at each of four middle schools will be replaced this fall by a two-tiered model, in which the top quarter of sixth graders will be enrolled in honors classes, the rest in college-prep classes. (A fifth middle school is a magnet school and has no tracking.)

More than 300 Stamford parents have signed a petition opposing the shift, and some say they are now considering moving or switching their children to private schools. “I think this is a terrible system for our community,” said Nicole Zussman, a mother of two.

Ms. Zussman and others contend that Stamford’s diversity, with poor urban neighborhoods and wealthy suburban enclaves, demands multiple academic tracks, and suggest that the district could make the system fairer and more flexible by testing students more frequently for movement among the levels.

But Joshua P. Starr, the Stamford superintendent, said the tracking system has failed to prepare children in the lower levels for high school and college. “There are certainly people who want to maintain the status quo because some people have benefited from the status quo,” he said. “I know that we cannot afford that anymore. It’s not fair to too many kids.”

Educators have debated for decades how to best divide students into classes. Some school districts focus on providing extra instruction to low achievers or developing so-called gifted programs for the brightest students, but few maintain tracking like Stamford’s middle schools (tracking is less comprehensive and rigid at the town’s elementary and high schools).

Deborah Kasak, executive director of the National Forum to Accelerate Middle Grades Reform, said research is showing that all students benefit from mixed-ability classes. “We see improvements in student behavior, academic performance and teaching, and all that positively affects school culture,” she said.

Aidan Bracken, left, and Evenson Andre in class.

Daria Hall, a director with Education Trust, an advocacy group, said that tracking has worsened the situation by funneling poor and minority students into “low-level and watered-down courses.” “If all we expect of students is for them to watch movies and fill out worksheets, then that’s what they will give us,” she said.

In Stamford, black and Hispanic student performance on state tests has lagged significantly behind that of Asians and whites. In 2008, 98 percent of Asian students and 92 percent of white students in grades three to eight passed math, and 93 percent and 88 percent reading, respectively. Among black students, 63 percent passed math, and 56 percent reading; among Hispanic students, 74 percent passed math and 60 percent reading.

The district plans to keep a top honors level, but put the majority of students in mixed-ability classes, expanding the new system from sixth grade to seventh and eighth over three years. While the old system tracked students for all subjects based on math and English scores, the new one will allow students to be designated for honors in one subject but not necessarily another, making more students overall eligible for the upper track.

The staff of Cloonan Middle School decided to experiment with mixed-ability classes for the last eight weeks of this school year.