Reviews of social comparison processes in school contexts have concluded that individual differences in propensity to compare with others are likely to be important in the classroom, but that there is a paucity of evidence testing this key proposition. To address this gap, we developed and validated a new measure of propensity to engage in academic social comparison (Motivation for Academic Social Comparison Scale, MASCS) among 11 samples of UK school pupils and two samples of university students (N = 1,991). The instrument showed good psychometric properties across samples. We then assessed the relationship between MASCS and actual academic performance in public examinations in a subsample of Year 11 and Year 13 students (n = 1,427). The MASCS positively predicted subsequent academic performance. Specifically, greater propensity to engage in academic comparison led to better objective academic achievement. This research thus establishes a valuable new measure and has meaningful implications, both theoretically and for education strategies.