National and state-based science and Science, Technology, Engineering, and Mathematics (STEM) initiatives expect educators to incorporate inquiry- and design-based practices into their STEM-centric lesson planning. While this is a new expectation and may be a challenge for all, it is an even more prohibitive barrier for elementary teachers who often find STEM outside of their comfort zones. Graduate coursework sensitive and attentive to elementary teachers' needs is essential to prepare teachers for these instructional demands. The Elementary STEM Instructional Leadership (ESIL) graduate program at McDaniel College, a small, liberal arts school in Maryland, developed such a program in conjunction with Carroll County Public Schools and the National Institute of Aerospace. The pilot cohort of elementary teachers for the ESIL program was the audience for this quantitative study. Lesson plans developed by this cohort were assessed to gauge the teachers' proficiencies in planning STEM-centric lessons. A proportion of agreement consensus and test of hypothetical value provided results that suggest that the sample population demonstrated proficient planning abilities for targeted standards. Findings from this research not only add to the field's knowledge of elements in the promotion of graduate coursework that leads to elementary in-service teachers' proficiency in planning STEM-centric lessons, but also have broader implications for teacher education at large. The McDaniel College ESIL model could frame flexible K-12 teacher education as it promotes analytical thinking and self-reflection.