This mixed methods study examined the impact of School Y's Montessori approach on their students' academic achievement, perceptions of executive functioning skills, and the school's culture. The purpose of this study was to determine the impact of length of enrollment on academic achievement in a Montessori upper elementary and middle school classroom. Administrator, parent, student, and teacher perceptions of the impact of School Y's Montessori approach on students' executive functioning were examined. The stakeholders' perceptions of School Y's culture were also identified. There were three main findings of the study: (1) there was no statistically significant relationship between the number of years students have been enrolled and their academic scores on both the Stanford Achievement Test and the OLSAT, (2) executive functioning skills were attributed to student success, and (3) cultural practices included individual instruction, mastery, real-life learning, positive discipline, peace education, a strong sense of community, parent education and support, mindfulness in the classroom, and an emphasis on environment. Additional findings showed the NCE results of both the Stanford and the OLSAT were above grade level expectations across all grade levels and subjects. These results could be an indication that the Montessori approach used by School Y was preparing the students for success in the subjects assessed. The findings were inconclusive because there was no statistically significant relationship between the number of years students have been enrolled and their academic scores on both the Stanford Achievement Test and the OLSAT. However, their results were above grade level expectations across all grade levels and subjects. [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://www.proquest.com/en-US/products/dissertations/individuals.shtml.]