South African mathematics teachers in the Senior Phase (Grades 8 and 9) were introduced to the pedagogical tool, meaning equivalence reusable learning objects (MERLO), as a formative assessment (FA) strategy to promote and support teachers' professional growth in using FA practices in the classroom. The cultural-historical activity theory (CHAT) and meta-didactical transposition (MDT) were used to frame the evolution process of teachers' praxeologies. In this study we used qualitative participatory action research that encompassed 3 phases: pre-MERLO phase, MERLO workshop and post-MERLO phase. The study was conducted in the northeast of Pretoria in the Tshwane district in the Gauteng province, South Africa. Twelve Senior Phase mathematics teachers were purposively sampled in 6 public schools before the workshop training. During the workshop training, only 5 teachers participated due to the COVID-19 outbreak. The data collection techniques included pre- and postinterviews, workshop training sessions, classroom observations, field notes, teachers' reflective journals, teachers' lesson plans, learners' workbooks and learners' worksheets, and data were analysed using thematic analysis. The findings reveal that the teachers acquired adequate knowledge and skills to effectively structure and integrate the lesson plan of teachers' didactical praxeologies as FA activities into their mathematics classrooms. The findings also reveal that the learners showed more interest and motivation, were actively involved, developed a deeper understanding of mathematics content, and showed increased autonomy in learning. Future research could involve implementing MERLO in all South African provinces and introducing MERLO to other emerging countries. However, the findings of this study are based on a limited sample of teachers and schools, and the recommendation is that, for future studies, more teachers should be involved in the MERLO professional development.