A portion of high-school students do not experience success or encounter extenuating circumstances that do not allow them to attend a traditional comprehensive high school to complete their high-school diploma requirements, so they enroll in some kind of alternative education to complete these requirements. These alternative education programs/schools offer instruction in a variety of formats such as group and individualized instruction to best meet the needs of their students. Based on Keller's Personalized System of Instruction theory, Vygotsky's Social Constructivism theory, Bandura's Social Learning theory and Kolb's Experiential Learning theory, this quantitative causal comparative study was used to examine the effects of Individualized Instruction (II) on Alternative Education (AE) students' standardized Math and English Language Arts (ELA) scores as compared to AE students enrolled in a group instruction (GI) program and then compared the alternative education students to students enrolled in a traditional high school using large group instruction. The impact of mediating variables of student perception of learning and perception of interaction with teachers and other students on the dependent variables were also examined. Both alternative education programs' test scores were compared to a traditional comprehensive high school all located in the same community in Southern California. ANCOVA statistical tests, independent t-tests and multiple regressions were utilized to examine these variables to provide the researcher with answers to the research questions. This research study revealed that there was a significant difference in achievement as determined by standardized English Language Arts and Math scores across the three different instructional approaches. However, the results did not fall in line with existing research. Existing research indicated that smaller class sizes in the primary grades improved achievement, but this research study revealed that students taught using this instructional approach did not experience improved achievement when compared to traditional instruction approach students. This study also examined all three instructional approach students' perceptions of learning, student interactions with instructors and other students. The researcher discovered that there was a significant difference across approaches. The alternative education students both group instruction and individual instruction students reported more positive perceptions of learning and teacher interaction than traditional approach students, which is aligned with existing research. While traditional instructional approach students reported more positive interactions with other students which may be a result of higher student numbers in the classroom. Lastly, students' learning perceptions, student interaction with instructors and other students were not found to mediate the achievement levels on the English Language Arts and Math test scores across the approaches. These findings go against Kolb's theory in terms of perception positively impacting achievement. Overall, this research indicated that there clearly are differences in achievement based on instructional approach which is significant, because different approaches prove valuable for different learners. However, the factors that impact achievement do not follow existing research that was conducted on primary grade students. This fact indicates that more research needs to be conducted to discover which factors greatly impact achievement at the secondary level. Also, since the alternative education students reported more positive perceptions than traditional students, researchers and educators should discover why this exists and if there is some way to improve these perceptions in traditional approach classes to see if this positively impacts student achievement. Educators continue to strive to find ways to improve learning and increase academic achievement. [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.]