The purpose of this quantitative causal-comparative study was to determine the extent of differences in student achievement scores on the ACT based on enrollment in preparation courses in large group settings, for students graduating from 2011 to 2020 at a private high school in the southwestern United States. Bandura's Social Cognitive Theory served as theoretical foundation for this study. A private high school in the southwestern United States provided de-identified, archival student data, including enrollment data for ACT preparation courses and student achievement scores on the ACT math, science, English, and reading sections for graduates from 2011 to 2020. Research questions investigated if statistically differences existed between mean ACT Math, ACT Science, ACT English, ACT Reading, and ACT Composite scores based on enrollment or non-enrollment in ACT Prep. Results of the one-way MANOVA revealed that there was no statistically significant difference between Enrolled and Did Not Enroll groups on the combined dependent variables of ACT Math, ACT Science, ACT English, and ACT Reading scores, Wilks' Lambda = 0.996, F(4,314) = 0.320, p = 0.864, [eta][superscript 2] = 0.004, (p > 0.05). Results of an independent-samples t-test revealed that there was no statistically significant difference between Enrolled and Did Not Enroll groups for ACT Composite scores, M = 0.338, SE = 0.518, t(317) = 0.652, p = 0.515, d = 0.076, (p > 0.05). This study did not support that ACT preparation courses make a statistically significant difference in mean ACT math, science, English, reading, and composite scores. [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.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]