Do streaming (mixed ability versus similar ability grouping) and retention both have undesirable impacts on students?

Tracking or streaming refers to whether classes are heterogeneous (mixed ability) or homogeneous (similar ability). In US, this is often called 'tracking'; in New Zealand and Australia this may be called 'streaming' or 'banding'; in United Kingdom 'setting' is another term commonly used. Studies consider both achievement effects and equity effects of this practice. The research shows tracking or streaming has relatively small effects (d=0.12) on learning outcomes but quite profound negative equity effects.

Retention is the practice of not promoting students up to the next grade/level in school when they fail to successfully complete the current grade/level requirements. Instead they repeat the same school level/grade again. The research shows that this is, on average, negatively associated with student achievement (d=-0.32). It is also negatively correlated with social/emotional adjustment, behavior, and self-concept. This is one of the few areas in education where it is difficult to find any studies with a positive effect. Even where two students have been matched for ability at the time of promotion, there are more positive outcomes for the promoted student than the retained student.