Large-enrollment, undergraduate college courses often use plenary reviews before exams. Alternatives such as no review, trivia games, or practice exams have been evaluated. We present a before-and-after comparison of a novel intervention to improve exam performance in an interdisciplinary, introductory ecology course enrolling 150–220 non-majors. We evaluated summative exam performance of 397 participants and non-participants across 3 exams after some students reviewed in ‘extended student hours’ of sequential student-led meetings with the instructor for >20 minutes per group of <8 students, compared to those using practice exams only. Using a repeated measures, within-subject Hills-Armitage ANOVA and grouped comparisons to detect main, dose, order, and carry-over effects, we found that 4 of 7 treatment groups averaged 73–78% before intervention and improved 7–14% over practice exam participants, whereas the other 3 treatment groups that averaged 83–88% beforehand did not change after intervention, without significant order effects or carry-over effects. We found the positive, dose effect was 1<2=3. We present an approach to minimizing self-selection bias. It is unclear if the content or the format of extended student hours explained the effects. The effect size was similar to reports for trivia game reviews. Extended student hours appear to aid in formative assessment before exams.