The purpose of this review is to systematically uncover relevant studies in the literature that measure the effects of class size on academic achievement. The review will synthesize the effects in a transparent manner and, where possible, the review will investigate the extent to which the effects differ among different groups of students such as high/low performers, high/low income families, or members of minority/non-minority groups, and whether timing, intensity, and duration have an impact on the magnitude of the effect. The primary focus is on measures of academic achievement. Academic achievement outcomes include reading and mathematics. Outcome measures must be standardised measures of academic achievement. The primary outcome variables are standardised literacy tests (e.g. reading, spelling and writing) and standardised numeracy tests (e.g. mathematical problem-solving, arithmetic and numerical reasoning, grade level math). In addition to the primary outcome, the review will consider school completion rates as a secondary outcome.