The Common Core State Standards (CCSS) have recommended that students increase reading of expository texts in order to be successful in both the university setting and in the workplace. Yet, frequently, the demands of expository text involve prior knowledge that might be limited for students of low socioeconomic status (SES). In this study, we examined student reading comprehension measures of student performance on items of expository (also called "informational") text. We analyzed data from a convenience sample of 1,339 participants taking English III benchmark assessments (fall, winter, and spring) from 42 high schools in the southeastern United States using trend analysis, followed by latent growth curve (LGC) analysis. We examined the trend of reading growth time and the relationship between student initial performance and growth on test items. Further, we examined the impact of SES on the relationship between students' initial performance and growth rate to determine the presence of Matthew effects (i.e., the gap between strong and poor readers increasing over time). Results detected non-linear, quadratic growth trends among medium- /high- and low-SES students, with both groups exhibiting similar patterns of growth during the school year. We did not detect Matthew effects present between the two groups, possibly owing to equalizing effects of instruction during the academic year that targets the needs of low-SES students while also adequately challenging students who are not economically disadvantaged.