Insights derived from classroom data can help teachers improve their practice and students' learning. However, a number of obstacles stand in the way of widespread adoption of data use. Teachers are often sceptical about the usefulness of data. Even when willing to work with data, they often do not have the relevant skills. Tools for analysis of learning data can, theoretically, aid teachers in data use, but often fall short of their potential as they are commonly designed without reference to educational theory and rarely consider end-user's needs. Keeping these challenges in mind, we designed a professional development program that aimed at, among other things, improving teachers' beliefs regarding data and their data literacy skills. After the training, we found that teachers had more positive attitudes regarding data. However, some data literacy skills proved quite difficult to learn. We present and analyse our intervention here and forward a proposal for improving the effectiveness of data use interventions by leveraging theory-based Learning Analytics (LA) dashboards as mediating tools that scaffold teachers' acquisition of new knowledge and skills during and beyond the intervention.