In this quantitative study, we compare the efficacy of Level 2, guided inquiry-based instruction to more traditional, verification laboratory instruction in supporting student performance on a standardized measure of knowledge of content, procedure, and nature of science. Our sample included 1,700 students placed in the classrooms of 12 middle school and 12 high school science teachers. The instruction for both groups included a week long, laboratory-based, forensics unit. Students were given pre-, post-, and delayed posttests, the results of which were analyzed through a Hierarchical Linear Model (HLM) using students' scores, teacher, level of school, Reformed Teaching Observation Protocol (RTOP) scores, and school socioeconomic status. Overall, compared to students in traditional sections, students who participated in an inquiry-based laboratory unit showed significantly higher posttest scores; had the higher scores, more growth, and long-term retention at both the high school and middle school levels, if their teacher had stronger implementation of inquiry methods (as measured by RTOP scores); and tended to have better outcomes than those who learned through traditional methods, regardless of level of poverty in the school. Our findings suggest that Level 2 inquiry can be an effective teaching approach to support student learning as measured through standardized assessments. (Contains 7 tables and 10 figures.)