The empirical material in this paper is from a Swedish upper secondary school where the mathematics lessons over the last two years have been co-taught. Co-teaching implies that two teachers are most often present in the classrooms during the mathematics lessons. Despite this additional support, students’ performance in mathematics remained low and this is why a professional development program was initiated. The aim of the professional development program was to find new ways to increase the number of approved students. At the start of this professional development program, classroom observations and a questionnaire were conducted with teachers and students. The results indicate that teachers’ and students’ views on good mathematics teaching became a limitation for the design of the co-taught lessons. Thus, to increase the number of approved students, teachers’ and students’ views on good mathematics teaching ought to be the focus of the professional development program.