Excessive workloads are a concern for teachers across the world. Overwork puts pressure not only on the physical and mental health of teachers, but also on their ability to provide quality education to their students. Nevertheless, many teachers struggle under the weight of excessive workloads, and have very little input in the top-down policies that impact their everyday working lives. In Japan, where overwork is a pervasive problem, teachers and their allies have been using Twitter to share their stories and advocate for more manageable working conditions, and this has coincided with an increase in mainstream attention to the issue. This paper presents a qualitative content analysis of highly engaged 'tweets', to identify how one form of social media is being harnessed to bring wider public attention to an otherwise neglected educational issue. The study has implications for teachers in other parts of the world looking to develop their agency through social media advocacy.