Dividing students by ability, into groups such as honors/regular/remedial, with different levels of challenge. https://en.wikipedia.org/wiki/Tracking_(education)

Tracking.

You break kids up by test score and then put all the top kids together, all the middle kids together, and all the bottom kids together. My public school experimented with it for one year, but reverted. Rumor was, parents felt like their kids were being written off in the 6th grade.