This study evaluates the effectiveness of an intervention of formative assessments with a clicker-based technology on anxiety and academic performance. We use a randomized experiment in physics education in one school in Dutch secondary education. For treated students, the formative assessments are operationalized through quizzing at the end of each physics class, where clickers enable students to respond to questions. Control students do not receive these assessments and do not use clickers, but apart from that, the classes they attend are similar. Findings from multilevel regressions indicate that the formative assessments significantly reduce anxiety in physics and improve academic performance in physics in comparison with traditional teaching. Furthermore, a mediation effect of anxiety in physics on academic performance is observed. In sum, this implies that an easy to implement technique of formative assessments can make students feel more at ease, which contributes to better educational performance.