At-risk high school students had low literacy and mathematics proficiency state test scores for the past 6 consecutive years in one suburban public school district in the Northern United States. In 2016, school administrators implemented supplemental education services (SES) to help these students to improve their performance on state tests. The purpose of this quasi-experimental research study was to examine if there was a statistically significant difference in literacy and mathematics state test scores before and after the implementation of SES into the curriculum. The theoretical foundation was Vygotsky's social development theory, which posits students make connections between the teaching environment and their experiences. With a convenience sample n = 227 at-risk high school students in Grades 10 and 11 from one suburban public school district, a comparison of 1,362 archived matched literacy and mathematics state scores before and after the implementation of SES was used to determine if there is a statistically significant change using a t test. After the implementation of SES, there was a statistically significant increase in the ELA and mathematics state test scores of at-risk students at the research site. The findings of this study may assist high school administrators to continue to use SES to assist at-risk students to reach proficiency on state tests. A positive social change includes the recommendation to continue to use SES at the research site for at-risk students to increase their state test scores in ELA and mathematics for these students to graduate from school and to enroll in postsecondary education, including trade schools and entering the workforce. [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.]