The implication is that less able students are disruptive. But in my experience, one reason that "bottom" sets contain disruptive students is because schools see them as a dumping ground for students who may actually be able, but bored.
Many students in bottom sets are hard-working and keen to improve. Also, "top" sets are often populated by laid-back, under-achieving students who feel they need do no more!
There is a further danger that setting makes teachers feel they do not need to differentiate work to meet individual students' needs, when any set can, in fact, contain a wide ability range.
Teaching mixed-ability groups can be difficult, but is extremely rewarding for all. It is important not to put a ceiling on what students can achieve.
In a mixed-ability set those able, but bored students in bottom sets could show what they can do.
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