Objectives: Although schools have an important role to play in preventing vape use, literature about teacher perceptions of vaping among students is limited. This study investigated teachers' perceptions of student vaping in Australian secondary schools over a 12-month period. Design: Mixed methods design. Setting: Australian secondary schools. Method: We used quantitative survey data from secondary school teachers for three waves of the Generation Vape project from March-April 2022 to March-April 2023 (n = 1,364), and qualitative focus group data from March-April 2023 (n = 6 groups, n = 32 participants). Differences in survey data across time were tested using multinomial regression and linear regression, and common focus group data themes were identified. Results: Eighty-five percent of teachers were concerned or very concerned about vaping at school, with the proportion who were very concerned increasing from 43% to 55% over the 12 months. There was consistent and strong agreement that vape use is unsafe for young people and high agreement that vaping during adolescence can cause addiction. Focus group discussions provided context to survey findings: teachers' awareness of and concern about vaping in schools, and the harms of vaping in students, which both then influenced school priorities for vaping management. Findings suggest inconsistent strategies to manage vaping in schools detract teachers from their core focus on teaching. Conclusion: Our study, the first to investigate teacher perceptions of vaping in Australian secondary schools across time, demonstrates increasing teacher concerns. As vaping is a community-wide problem, a coordinated response across government is required to support teachers in reducing student vape use at school.