The purpose of this study was to examine the effect of selected demographic factors on the academic achievement of high school students. More specifically, this study was concerned with the effects of the variables gender, ethnicity, and socioeconomic status on the State of Texas Assessments of Academic Readiness (STAAR) scores in Mathematics, Science, English, and Social Studies.A quantitative causal-comparative or ex post facto research design was used in this study. The population for this study consists of high school students enrolled in a public school in the state of Texas during the 2019-2020 academic school year. The sample population for the current study consisted of 222 high school students enrolled in a suburban public school in the State of Texas during the 2021-2022 academic school year.The State of Texas Assessments of Academic Readiness (STAAR) was the investigative instrument used to collect the data. The raw scores data was used to measure the academic achievement of high school students in math, English, science, and social studies. The One-way Multivariate Analysis of Variance Statistical procedure was utilized to analyze the data for this study. All three null hypotheses were tested at the 0.05 level of significance or better. From the results, this study concludes that gender did not affect the combined STAAR's Mathematics, Science, English, and History scores of high school students. Regarding ethnicity, White and Asian high school students scored significantly higher on the Mathematics and Science sections of the STAAR examination than Black and Hispanic high school students. High school students who received free or reduced lunch scored significantly higher on the STAAR's Mathematics, Science, English, and History sections than those high school students who did not receive free or reduced lunch. [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.]