The main purpose of the present study was to investigate whether there is a crossover effect from teacher's general (trait-level) work engagement to their pupils' weekly positive affect in school and to examine whether pupils' weekly autonomous motivation for school functions as an underlying mechanism that may explain this crossover effect. Building upon the self-determination theory and the emotion contagion theory, we argue that teachers' general work engagement can be a strong resource for pupils, which can foster their psychological investment and interest in school-related activities, skills, and tasks (i.e. autonomous motivation), and, in turn, their positive affect. To test our hypotheses, we employed a weekly diary methodology by following 50 teachers and their 916 pupils in six different countries for three consecutive work weeks, which yielded 2735 reports from pupils and their teachers. The results of multilevel modeling provided support for the shypothesised research model. When teachers were generally more engaged in their work, their pupils reported more weekly positive affect in school, and this positive crossover effect was mediated by pupils' weekly autonomous motivation for school. These findings extend current literature by revealing the potential underlying mechanism that can explain how teachers' work engagement transmits to pupils' positive affect in school.