Assessment and evaluation are at the heart of all functioning systems, as progress can be determined with the information collected by these assessments. The problem addressed in this study is the lack of actionable data to evaluate whether assessments aligned with Montessori benchmarks support the achievement of student outcomes. To address the problem, this quantitative expo facto causal-comparative study explored the effects of Montessori benchmarks with aligned assessment on student outcomes. As required by the American Montessori Society, system evaluation must occur to ensure alignment with the Montessori framework. The study used a total population sample of students in grades three through six at a Montessori School in the Midwest United States. The materials included 209 Comprehensive Test of Basic Skills reading and math composite scores collected before and after the implementation of benchmarks. A statistically significant difference between the two groups was found in reading and math test scores, showing that the scores collected after the benchmarks were implemented had significantly lower scores than those collected before the benchmark implementation. This finding was confounded by the unknown effects COVID-19 protocols may have had on learning loss. Implications drawn from the findings include alignment of benchmarks to teaching practices and continuity of delivery of teaching practices, with the recommendations for practice including evaluating the teachers' understanding of the benchmark system, teaching materials and strategies, inter-rater reliability of teacher observation and assessment, and evaluation of the assessment tool to determine alignment. Continued research could benefit by using data from schools that implemented benchmarks pre- or post-pandemic and broadening the study to include other curricular areas such as language, spelling, science, and social studies could add to the robustness of the study. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]