This paper presents results from an examination of the relationships between high school (HS) school day length and 2011 New Jersey High School Proficiency Assessment (HSPA) Math and Language Arts Literacy test results. Variables found to have an influence on standardized test scores in the extant literature were evaluated and reported. Hierarchical regression models were used to determine the strength of the predictive influence of these variables, specifically school day length, on both HS Math and Language Arts Literacy student performance. Results indicated that school day length does not have a significant influence on HS LAL achievement, but accounts for 1.8% of the variance in HS Math achievement scores. Implications for practicing administrators are discussed.