In spite of its growing popularity, researchers have focused little attention on the effectiveness of combining traditional classroom instruction and online learning, a practice generally referred to as blended learning. The modest research on blended learning to date has tended to focus on higher education, leaving a significant gap in the research regarding K-12 education. Even less attention has been given to blended learning in charter school K-12 education. Framed within Vygotsky's theory of social development, the purpose of this causal-comparative research study was to determine if there were any significant differences when comparing charter school students who participated in a blended learning approach to reading and math with students who studied the two subjects in fully online classes and with students who studied them in traditional classrooms with no online learning. The design was causal-comparative with a nonrandomized control group. The study compared the archived 2014 State of Texas Assessments of Academic Readiness scores of 1797-2298 students in one charter management organization: students in a blended learning environment, students who received traditional classroom instruction, and students who used fully online learning. Analysis of Variance (ANOVA) was used in combination with appropriate post-hoc comparisons to evaluate group means. The study determined that there is not a statistically significant relationship between traditional, blended, and fully online students and math scores, nor between traditional and blended learning students and reading scores. However, there was a statistically significant relationship between fully online students and higher reading 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/en-US/products/dissertations/individuals.shtml.]