Recent standardized state tests have revealed a consistent decline in reading scores across grade levels, forcing conversation about intervention in place to combat this. Developed using Vygotsky's theory of Zone of proximal development, Houghton Mifflin Harcourt's Read 180 reading intervention program promises to close reading gaps of up to two years using its prescribed curriculum. The problem in this study was that many high school students are heading toward graduation without reaching grade-level proficiency in reading (Richards, 2020) despite research-based remedial practices in place to combat this. The purpose of this quantitative, quasi-experimental, casual comparative study is to evaluate to what extent the Read 180 intervention program had on ninth and tenth-grade students whose Lexile level was between 850L- 1099L - the average reading level of American adults - compared to those students within the same Lexile range receiving only traditional English language arts instruction. Reading Inventory pretest and post-test scores were collected during one full school year and analyzed using an independent samples t-test. Results showed no significant effect on Lexile growth scores of those students who participated in the year-long Read 180 reading intervention coupled with traditional English language arts instruction compared to those students who only received traditional English language arts instruction. Findings in this study revealed higher Lexile growth averages in posttest scores of students only receiving traditional English language arts instruction. Recommendations for future research include qualitative studies on the reading motivation of struggling readers who are both academically successful and unsuccessful, as well as teacher attitudes and motivation about fidelity in implementing scripted intervention programs. Further quantitative research recommendations are to analyze the growth of more diverse populations. [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.]