The No Child Left Behind legislation of 2001 requires all public school students to be proficient in math and English by 2014. The research problem that this study addressed is that the majority of schools are not on track to demonstrate this proficiency, potentially creating a nation of schools designated as "failing." This problem has underscored the importance of new and innovative approaches, both during and after school. The purpose of this study was to investigate the effectiveness of a particular approach, a computer-based afterschool intervention program designed, to deliver differentiated instruction to low-performing students. The research questions for this study concerned the effectiveness of this treatment in improving overall and reading comprehension scores on English district benchmark exams. The theoretical foundation for this study was Vygotsky's zone of proximal development, according to which students learn best when they receive individualized instruction delivered at their academic level. This quasi-experimental study used a regression discontinuity design because it allowed for assigning those students who needed the treatment the most while yielding valid results. Linear regression was used to test for a discontinuity in the regression lines on either side of a predetermined cutoff value, the value that determined whether or not the participants were in the treatment or comparison groups. The discontinuity that was found represented a statistically significant treatment effect and supported the notion that delivering instruction in a student's zone of proximal development improves comprehension and overall academic performance. These results give teachers, administrators, and policymakers a research-backed strategy to address the achievement gap that still exists between urban and suburban schools, specifically in the area of reading comprehension. [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.]