Background: On the 23rd of November 2020, 14 days after the full re-opening of schools, a student from a boarding school in Mashonaland east province had an influenza-like illness which was confirmed as COVID-19. By the 9th of December 2020, 103 confirmed cases of COVID-19 had been diagnosed at the school. We determined factors for contracting COVID-19 in the boarding school. Methods: We conducted an unmatched 1:1 case-control study at the boarding school in December 2020. A case was defined as any student with a positive COVID-19 PCR test. Data were collected using interviewer-administered questionnaires, checklists and observations. Epi Info 7 was used to compute frequencies, proportions, means, medians and odds ratios. We calculated student density, reproductive rate and assessed knowledge of COVID-19. Results: We interviewed 82 case-control pairs. Form one students were mostly affected 69/77 (89%) of the student cases. This was a propagated outbreak with a reproduction number of 4. Cases 68/82(83%), and controls 75/82(92%), had good knowledge of COVID-19 (p=0.12). Sharing utensils and belongings was an independent risk factor for contracting COVID-19 (aOR=4.01 95%CI, 1.56-10.31). There was a student density of 0.67m²/ student in the classrooms and 2.2 m²/student in the dormitories against the recommended 1.5m²/student in classrooms and 3.75m²/student in the dormitories. Conclusion: Sharing of utensils and belongings amongst students and overcrowding in the classrooms and dormitories were the risk factors of this outbreak. Quarterly health inspections and recruitment of 38 students/class for forms one and five for 2021 were recommended. As a result of this investigation there was immediate closure of the school for non-public examination classes.