Proponents of grouping argue that without it, teachers are forced to teach to the middle, leaving out both struggling children and gifted learners. They also say there is a “peer effect,” in which high-achieving children do better if paired with other high-achieving students. Done judiciously and flexibly, they say, grouping can help all students. The reasons for the resurgence are unclear. Some experts attribute it to No Child Left Behind, the 2001 law that strengthened accountability standards for schools. By forcing teachers to focus on students who fell just below the proficiency cutoff, the law may have encouraged teachers to group struggling students together to prepare them for standardized tests.

Technology might have also played a role, Mr. Loveless said, with teachers becoming more comfortable using computers to allow children to learn at different speeds.

In interviews, several teachers said they believed modern-day grouping was not discriminatory because the groups were constantly in flux. But they acknowledged the additional challenge of tailoring instruction to different groups, as they must produce multiple lesson plans and keep closer track of students’ progress.

At Public School 156 in Brownsville, Brooklyn, which enrolls mostly African-American and Hispanic children, many living in homeless shelters, Cathy Vail randomly sorts her fifth graders at the beginning of the year using lettered sticks. After six weeks of testing and observing them, she shifts them into “teams” of seven or eight.

Children may be assigned to different groups for reading and math, and can switch groups if they have shown progress, struggle to get along with other students in a group or need extra help with a particular lesson. Ms. Vail uses thrice-yearly reading assessments and a test before each math unit to make sure children do not remain in groups that are too advanced or too slow for them, she said; one student this year, for instance, has moved up two groups in both reading and math.

Ms. Vail teaches the same lesson, whether it is a math concept or a book, to the entire class, but gives each group a different assignment. Working on each week’s set of new vocabulary words, all four groups draw illustrations and write captions using the assigned words, but she encourages team C, her highest-achieving group, to write more complex sentences, perhaps using two new vocabulary words in the same sentence. She also asks children in team C to peer-teach students in the other groups.

“At the end of the day, they’re learning the same words, but just with different levels of complexity and nuance,” she said.