Montessori is a highly individualized pedagogy that prioritizes following each child’s individual interests over standardized test scores. This quasi-experimental study compares groups of Montessori students (treatment groups, N = 535–710) with control groups of non-Montessori students, matched through the simple matching method, which controls for the effect of background variables on schooling outcomes. The tested hypothesis is that the treatment groups achieve mean scores that are not significantly lower and have a significantly higher standard deviation on standardized tests of mathematics and language. The study encompassed students in Grade 2, 5, 8, and 10 across two separate cohorts. Results indicate that Montessori students attain either comparable or significantly higher scores than non-Montessori students, depending on grade and cohort, with greater score variability observed, particularly at Grade 8 and in mathematics. This finding implies the necessity for future investigations into the effect of Montessori teachers’ instructional practices.