This study examines the relationship between English Language Learners' (ELLs) English proficiency and standardized test performance. Utilizing a mixed-methods approach, it incorporates quantitative data from test scores and qualitative data from teacher surveys and interviews. Findings indicate a significant correlation between ELLs' proficiency levels and their test outcomes. Furthermore, teachers with higher self-efficacy are more successful in preparing ELL students for standardized assessments. The study concludes with recommendations for improving ELL instruction through professional development and culturally responsive teaching practices to enhance academic success and educational equity for ELL students. [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.]