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.