Marxist disapproved of society which is being reproduced. In any way, Marxists against the idea of ability groupings. They think that would encourage inequality. Teacher may divide the groups base on their own observations, but it is hardly avoid the prejudice that is involved. Besides, once the label is attached on the student, it is very difficult to take it away.
Students have to live with the labels no matter what and how they perform.
It is especially difficult for the low ability learners to move up. It is sort of like a self-fulfilling prophecy . Once teachers think certain students are smart, they would construct certain expectations for these students and help them to achieve these goals. Therefore teachers are fulfilling their own prophecies. On the other hand, much of research in ability groupings show that children in low ability groups receive less instruction than high ability groups.