Mathematics teaching in a 21st-century educational context requires a shift to active learning. The shift calls on teachers, and the plans countries need to prepare to make the changes through well-designed teacher professional development programs. ARPA (Spanish acronym for "Activating Problem Solving in Classrooms"), as a professional development program, with an emphasis on collaborative problem solving, provides opportunities and tools for teachers to enrich their classroom activities in a student-centered manner. This study of 149 in-service teachers evaluates the impact of an ARPA workshop on teachers' self-efficacy and their tendency to choose student-centered practices. The results show that before participating in the program, even if teachers had believed in their ability to solve problems, they did not consider how to connect this ability with the active student-centered problem-solving practices in their classroom. However, teachers' self-efficacy in performing problem solving significantly affects the teachers' report of their student-centered practices after participating in the program. Understanding the relationship between teacher confidence and the quality of their classroom activities is essential for the optimization of the outcomes of any teacher's professional development.