In Sweden, there has been a development of continuously more and earlier mandatory assessments in schools in the last decades. One of the latest is mandatory assessment in preschool classes from 2019, with one of the purposes to identify students in need of Special Education in Mathematics (SEM students). Preschool class is a compulsory school year for 6-year-olds which precedes compulsory school. The study at hand explores how teachers create knowledge of mathematics and the SEM student during their work with this mandatory assessment. Results show that mandatory assessment might affect how teachers assess knowledge, develop their teaching, and (re)produce knowledge of the six-year-olds as knowers and of knowledge in mathematics. We join the vast amount of research that has shown that mandatory assessment is a potentially sensitive situation for students who are already unprivileged, who face obstacles in their learning or who struggle with mathematics. Awareness of processes of inclusion, exclusion and (in)equity during assessment and in the following education is central to not count (SEM) students out as competent learners with valuable knowledge.