The purpose of this quantitative, ex post facto study was to determine the difference in students' LEAP ELA achievement scores in grades 3-5 based on the teacher's level of dyslexia knowledge. This study used the Phonological Deficit Theory (PDT) and the Criterion Referenced Assessment Theory (CRT) as its foundation. PDT describes the levels of dyslexia and can be seen in the sections measuring teachers' knowledge in the Dyslexia Knowledge Questionnaire 2 (DKQ2) survey. CRT is represented in the archival data of students' ELA state assessment scores provided by the school district where this study was conducted. There were 3 research questions that asked what is the difference in students' LEAP ELA achievement scores in grades 3, 4, and 5 based on the teacher's level of dyslexia knowledge. This study was conducted in a public school district of Louisiana. Archival data were collected from 456 students whose 12 teachers completed the (DKQ2) survey developed by Peltier et al. (2022). Two groups were created using a median split of teachers' level of dyslexia knowledge. Independent samples t-tests revealed that grade 3 and grade 5 students with teachers with a high level of dyslexia knowledge had significantly higher ELA scores (grade 3: t (183) = 3.58, p = 0.001; grade 5: t (82) = 9.20, p = 0.001). After applying the Bonferroni correction to the alpha level, an independent samples t-tests revealed no significant difference in grade 4 student ELA scores based on the teacher's level of dyslexia knowledge t (185) = 1.98, p = 0.049. This study has implications for professional development and training for educators, which entails instruction on the science of reading and refuting misconceptions regarding dyslexia. [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.]