Teachers mixing in-class and remote online learning activities face numerous challenges in monitoring learners' online activity and progress, especially when preparing in-class interventions. We present a design study of PROGDASH, a dashboard enabling teachers to monitor learners' activity on an online grammar and spelling learning platform. We designed PROGDASH based on interviews with seven teachers to understand their needs, and we collaboratively iterated on design prototypes. We conducted a three-month field deployment with 17 teachers, collecting logs, diaries, and interview data to evaluate how PROGDASH informed their practices. Our findings extend prior work on teachers' practices using dashboards: teachers found PROGDASH actionable to devise informed pedagogical practices: monitoring, conducting assessments, planning interventions, sharing in-class, providing debriefing and feedback. Based on our findings, we suggest directions on how dashboards could effectively support teachers in schools using online learning platforms in authentic settings.