The introduced policy of “synchronized enrollment of public and private schools” has once again triggered the debate on the effectiveness of public and private schools. Based on the data of the Program for International Student Assessment (PISA) 2018 from four Chinese provinces and municipalities, this paper explores whether private schools gain a relative advantage in student academic performance through student “screening” or academic “meritocracy,” through a hierarchical linear model (HLM) and an empirical test of the propensity score matching (PSM). It has been found that the academic performance of students in private schools is significantly better than that in public schools. But with background, metacognitive ability, and learning hours of students in private schools controlled for, such academic performance is not significantly superior, suggesting that private schools rely heavily on student “screening” to achieve a relative advantage in student academic performance. This finding has also verified the scientific nature of the above policy.