Objective To investigate an outbreak of norovirus GII gastroenteritis in a school, and explore the causes, transmission routes and risk factors of the outbreak, and to provide a scientific basis and experience for effective control of outbreaks of acute gastroenteritis in schools. Methods A case study of an acute gastroenteritis outbreak in a school in Wujin District, Changzhou was conducted by field epidemiological investigation. Anal swab samples of patients and controls were collected, and enterovirus nucleic acid was detected by real-time fluorescent PCR. Results From December 12 to December 17, 2019, a total of 185 cases were identified, all of them from students of the school, with an incidence rate of 3.84% (185/4,822). All of the cases were distributed in three (7^th/8^th/9^th) grades of the school, with attack rates of 5.38% (31 / 576), 18.71% (104 / 556) and 9.06% (50 / 552) in 7^th, 8^th and 9^th grade, respectively, showing a significant difference among the grades (χ²=54.47，P＜0.05). The attack rate of each floor of the teaching building from the first to fifth floor was 2.17% (5/230), 7.51% (26/346), 15.77% (53/336), 17.11% (65/380), and 6.38% (25/392), respectively, showing a statistically significant difference among different floors (χ²=55.66，P＜0.05). A total of 32 anal swabs were collected, including 12 patients, 10 control students and 10 workers in the canteen. Of them, 9 specimens of the patients and 1 specimen of the workers were positive for norovirus type GII. Conclusion The school outbreak of acute gastroenteritis was caused by norovirus GII. Close contact between students was the main mode of transmission. A hidden infection of canteen staff may be related to this outbreak. It is important to promote popular science of norovirus prevention and to enhance the awareness of the risks of the virus.