This paper reports on a quasi-experimental study examining the effectiveness of flipped instruction in a 9th grade biology classroom. This study included four sections of freshmen-level biology taught by the first author at a private secondary school in the Pacific Northwest. Using a block randomized design, two sections were flipped and two remained traditional. The quiz and posttest data were adjusted for pretest differences using ANCOVA. The results suggest that flipped instruction had a positive effect student achievement, with effect sizes ranging from +0.16 to +0.44. In addition, some students reported that they preferred watching video lectures outside of class and appreciated more active approaches to learning.