Originally Posted by master of none
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... 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.
The above is just one quote from the article. This isn't really ability grouping. This is what our school system supports. Differentiated learning where the SAME lesson is taught to everyone, and then in small groups, they work on a very basic or a higher level on that SAME concept.
Agreed. These practices are a veritable "wolf in sheep's clothing", dressed to look like something acceptable and when welcomed capable of predation.

Consider the impacts on growth in pupil learning... if a pupil is dedicating time to teaching others rather than to their own growth and acquisition of skills, how might this be holding them back? ... leveling them out? ... plateauing? ... creating underachievement?

Consider the impacts on grading. In what ways is the expectation or rubric for Team C distinguished from others in the class, while being transcripted as the same course? For high school students, how might this play out in grade point average? In class rank?

True ability grouping... clustering by readiness and ability... allows pupils of any age or grade level to study curriculum at the appropriate challenge level and pacing... with their intellectual peers... keeping them working in their zone of proximal development... while not being socially isolated, nor lacking teacher instruction (possibly with the teacher as a "guide on the side").