Vivian Yee:

It was once common for elementary-school teachers to arrange their classrooms by ability, placing the highest-achieving students in one cluster, the lowest in another. But ability grouping and its close cousin, tracking, in which children take different classes based on their proficiency levels, fell out of favor in the late 1980s and the 1990s as critics charged that they perpetuated inequality by trapping poor and minority students in low-level groups.
Now ability grouping has re-emerged in classrooms all over the country — a trend that has surprised education experts who believed the outcry had all but ended its use.
A new analysis from the National Assessment of Educational Progressa a Census-like agency for school statistics, shows that of the fourth-grade teachers surveyed, 71 percent said they had grouped students by reading ability in 2009, up from 28 percent in 1998. In math, 61 percent of fourth-grade teachers reported ability grouping in 2011, up from 40 percent in 1996.
“These practices were essentially stigmatized,” said Tom Loveless, a senior fellow at the Brookings Institution who first noted the returning trend in a March report, and who has studied the grouping debate. “It’s kind of gone underground, it’s become less controversial.”

We have seen this movie before English 10.
Much more on ability grouping, here.