Statement of Purpose and Method of Study: Historically, Mississippi has lagged behind the rest of the nation in student achievement and funding for education. This comparative quantitative study investigated whether significant differences in student achievement as measured by mean composite ACT scores of high school juniors in Mississippi public schools existed as a result of differences in federal, state, and local school funding source percentages. This study compared the differences in mean ACT scores of high school juniors in each Mississippi public school district over a three-year period based on per-pupil instructional cost expenditures and percentages of local, state, and federal funding levels used in per-pupil instructional cost expenditures to fund student achievement. This study analyzed whether significant differences existed in academic achievement measures of mean composite ACT scores among school districts classified as Above Average, Average, and Below Average in relation to per-pupil instructional cost expenditures (PPICE) in the 2015-16, 2016-17, and 2017-18 school years. Comparisons were made between mean ACT scores of juniors from the divided per-pupil instructional cost expenditure and funding source groups of school districts. Findings and Conclusions: Statistical analysis included data from 144 school districts and revealed both statistically significant and insignificant differences in mean ACT scores for public school juniors in Mississippi based on instructional costs and percentage of funding allocation. The findings of this study may provide support for modifications to policies guiding specific designations of per-pupil instructional costs. [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.]