Adam Gamoran, of the centre for education research at Wisconsin University, told a seminar in Edinburgh: "We demand competing goals. Tension will always exist between securing achievement and creating inequality."
The general performance of secondary pupils increased where some were put in higher-ability classes, Professor Gamoran said, but there was a depressing effect on the lower streams. There was therefore no case for streaming or setting which emphasised a distinction in status between classes. Still less was there a case for a return to selective schools, as was being suggested in England.
Professor Gamoran, who has examined evidence from both Scotland and the United States, suggested that there tended to be a difference between teaching and learning in classes of higher and lower ability. The American experience was that abler classes were allocated to more experienced teachers who interacted more with their pupils and expected rigorous application and a higher level of attention.
In English, for example, lower ability classes were confined to "young adult" books such as those by Judy Bloom.
The result is "polarisation with top students becoming more responsive whereas the less able were turned off". Professor Gamoran speculated whether selection had already singled out alert and attentive pupils for able classes.
Streaming was more damaging than setting, he claimed, because matching abilities across a range of subjects was less exact than for an individual subject. The stigma of the lower stream was greater.
But Professor Gamoran warned that it would be "a mistake to mandate all schools" to follow a particular type of class organisation. Decisions should be made school by school, or even teacher by teacher.
In Scotland, where classes were usually of mixed ability, pupils reported that some work was too easy. Interestingly, that view extended to less able as well as able pupils. Only in the Standard grade years did pupils cease to complain that lessons were too easy.
Cameron Harrison, chief executive of the Scottish Consultative Council on the Curriculum, said that teachers as well as pupils reported underachievement in the lower secondary. He suggested that mixed-ability teaching worked less well in secondary than in primary because pupils have some teachers for only 50 minutes a week in the first two years.