Modern classrooms are indeed getting high-tech. Smartboards replace whiteboards. Laptops and iPads are needed in the classroom to comply with inquiry-based learning. The problem investigated in this quantitative causal-comparative study was that although the technology is more prevalent than ever before, the leaders of school districts are struggling to implement appropriate policies for the usage of smart devices in high school classrooms. The purpose of this quantitative causal-comparative study was to inform educators and leaders of school districts about the influence of smart devices on high school students' performance on standardized assessments and inspire them to develop and or modify their smart device policies. The conceptual framework that was the basis of this research study is the pedagogy and andragogy associated with blended learning. The study was guided by two research questions that helped to analyze the paper in detail. This study's primary material was the archived scores of students on Smarter Balanced Assessments of two high schools in the same community in the Los Angeles area. One of the schools incorporated a zero-tolerance policy in the use of smartphones on their campus, including classrooms, whereas the other school did not restrict smart devices in classrooms. The data were collected and compared through a two-tailed independent sample t-test and confirmed through a two-tailed Mann-Whitney U test. The outcome of both tests indicated that a statistically significant difference existed in the mathematics and English performance on Smarter Balanced Assessments between students in schools using smart devices and schools that did not use smart devices in class. The use of smart devices during instruction negatively influenced the academic achievements of high school students in mathematics and English. So, during instruction in mathematics and English classes, students should not use any personal smart devices and, if needed, use only the district-issued smart devices because those devices block access to all social media. Researchers can replicate this study in different states and even other countries to affirm or possibly refute the findings of this study. Future researchers can also conduct a quasi-experimental study on three groups of 60 students, each from mathematics and English classrooms, to determine the best practices regarding the usage of smart devices in classrooms. [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.]