This study investigates how genre can be used as an organisational principle to interweave teaching and assessment in the L2 school context. Relying on data from interviews and lesson observations gathered from two Secondary 1 (that is, Grade 7) Hong Kong classrooms, the study sought to discover how teachers implemented genre-based teaching and assessment in traditional product- and exam-oriented writing classrooms, students' and teachers' evaluation of the approach, as well as the factors that might facilitate or restrain the genre innovation. The findings show that teachers worked collaboratively and shared a common vision in implementing genre-based teaching and assessment, making sure that they integrated teaching and assessment through a focus on genre. Although the limitations of the genre approach were noted, the innovation was generally well received by students and teachers, who felt that the innovation had enabled students to improve their writing and helped teachers enhance their practice. A few facilitative and inhibitory factors are discussed. The findings suggest that the implementation of genre-based teaching and assessment has to be considered within the ecology of teachers' work.