This study investigates the impact of a professional development programme in formative assessment (FA) for high school mathematics teachers. Through a collaborative approach between researchers and in-service teachers, a 16-week online course was designed focusing on feedback strategies, peer co-assessment, and student engagement. A mirrored scale was used where teachers and students reported frequency of FA practices. Pre-post tests were used to measure FA knowledge and practices in teachers (n = 110), as well as their self-efficacy. In students (n = 1029), mathematics learning and FA practices implemented by their teachers were evaluated. The results highlight a significant moderate effect on teachers' self-efficacy, and large effects in FA knowledge, and implementation of FA practices, with noticeable changes perceived by both teachers and their students. Students perceived larger improvements in collaborative practices and peer assessment and weaker changes in practices that involved them in self assessment. Additionally, a transfer of these changes to students' mathematics learning was observed, indicating that students' reports of FA practices significantly impact their mathematics performance. This study underscores the importance of involving students in the FA process to improve mathematics learning.