Objective: The asthma control test (ACT) is a validated tool for assessing control in asthmatic children aged 12 years and older. Using the ACT, we sought to assess asthma control and knowledge in London secondary school children. Methods: Secondary schools in London, UK, participated in this study. Children with doctor-diagnosed asthma were invited to complete an online questionnaire that included the ACT and questions about asthma. Suboptimal asthma control was defined as an ACT score of ࣘ 19 out of a maximum score of 25. Data are summarised as median and interquartile range (IQR), and were analysed by either Mann-Whitney test, or chi-square test. A p value of < 0.05 was considered significant. Results: A total of 799 children completed the questionnaire; 689 (86.2%) were included for analysis. Suboptimal asthma control was reported by 49.6% of students. Over a third (42.4%) of students prescribed a short-acting β2-agonist inhaler felt uncomfortable using it at school, and 29.2% (n = 173) reported not using this inhaler when wheezy. 56.4% (n = 220) of those with regular inhaled corticosteroids did not take them as prescribed, and 41.7% did not know what this inhaler was for. Suboptimal control was associated with a greater proportion of students reporting that they were 'somewhat' , 'hardly' or 'not at all' comfortable using inhalers at school (52.7% vs 29.1%, p < 0.01) and outside school (22.8% vs. 14.8%, p < 0.01). Conclusions: Suboptimal asthma control and poor asthma knowledge are common in London schoolchildren.