School closure along with mass prophylactic oseltamivir treatment of pupils have been used in England and elsewhere to contain school outbreaks of influenza A(H1N1)v. We evaluated the protective effect, compliance with and side effects of oseltamivir chemoprophylactic treatment with a ten-day course of 1x 75mg given to 11-12-year-old pupils in one school year in a secondary school in South West England closed for ten days in response to a symptomatic laboratory-confirmed pupil. We distributed a questionnaire to pupils in the affected school year in class after the school had re-opened. Questions included symptoms of flu-like illness, compliance with chemoprophylaxis and side effects. All present on the day, 248 (93.2%) participated. Compliance with chemoprophylaxis was high, 77% took the full course, 91% took at least seven days. Fifty-one percent experienced symptoms such as feeling sick (31.2%), headaches (24.3%) and stomach ache (21.1%). Although some children were ill with flu-like symptoms, those tested did not have A(H1N1)v infection. Compliance with oseltamivir chemoprophylaxis was high, although likely side effects were common. The burden of side effects needs to be considered when deciding on mass oseltamivir chemoprophylaxis in children especially given that the symptoms of A(H1N1)v influenza are generally mild.