At a first glance, exam school students do well on most measures of achievement. This may be why many parents dream of sending their children to top exam schools. However, while exam school students clearly excel in school, the question remains: Does receiving an exam school education add value relative to receiving a standard public education for already high achieving students? To answer this question, SEII researchers employed a research design which compares test scores of the marginal applicants – those who fall just above and just below admission cutoffs. Using this design, the study estimates the causal effect of exam school attendance on student achievement as measured by standardized state test scores. The estimates show little effect of exam school offers on most students’ achievement in most grades. Students who win these prized seats might have benefitted from exposure to a higher-achieving group of peers and a more demanding curricula, but the results indicate that in spite of these factors, the students just above the admissions cutoff do not reap these benefits in the form of improved test scores. While there are a handful of small gains, the intense competition for exam school seats does not appear to be justified by improved achievement for most students.
http://seii.mit.edu/research/study/do-elite-exam-schools-add-value/