Considering that learning belongs primarily to the student, student self-regulation capacity is essential to ensure mathematics learning. Poor self-regulation limits students' mathematical performance and their ability to participate fully in classroom discourse. By enhancing self-regulation it is possible to improve students' equitable participation in mathematics. In this paper, we present a study aimed at understanding the development of the self-regulation capacity of middle school students in mathematics, throughout an observed teaching intervention, in which is developed regular work to promote internalisation of the assessment criteria, whole-class mathematics discussions regulated by social norms and the assessment criteria, and written self-assessments tasks. Following qualitative research methodology, data collection sources were classroom observation, interviews, questionnaires, and collection of documents. Data analysis had a strong interpretive foundation, making use of a system of categories defined during the process, based on the theoretical framework. Throughout the teaching intervention, students walked towards the internalisation of the assessment criteria, improved their performance in collective discussions, and developed their self-regulation capacity, following an individualized and non-linear pathway. Although equity concerns were partially addressed, the development of self-regulation capacity was not consistent in all students. This suggests that more could have be done to provide quality mathematics education for each and every student.