Recent studies on professional development programs indicate these programs, when sustained, have a positive impact on student achievement; however, many of these studies have failed to use longitudinal data. The purpose of this study is to understand how one particular instructional practice (STEM PBL) used consistently influences student achievement. Three urban high schools in the southwest U.S. were assigned to participate in a professional development program for STEM teachers. Data for 565 students from these schools were collected at grade 8, 10, and 11. According to instructional practice observation tool, School 1 had a high level of STEM PBL implementation, school 2 showed an average level of STEM PBL implementation, and school 3 had almost no STEM PBL implementation. Results indicated students' success for school 1 was significantly different than students' success for schools 2 and 3. Results also indicated no difference in student data for ethnicity in school 1.