Basic mathematical fluency is a prerequisite for success in a wide array of areas in biology and it has been noted that many students are deficient in this skill. In this paper, the use of a simulated peer-assessment activity is investigated as a method to improve performance in numerical problem-solving questions in high school biology. Additionally, the benefits of using simulated, rather than real, student’s answers in peer-assessment is discussed. The study involved a small cohort of students who carried out a simulated peer-assessment as a classroom activity and their improvement in performance and attitude towards the activity was measured. The results demonstrate that a simulated peer-assessment activity is suitable as a replacement for standard peer-assessment and that students’ attitudes favour the simulated approach.