Functions are one of the basic topics taught in mathematics curriculum at Secondary school level requiring knowledge from the students' past, and uniting mathematical topics. Mathematics teachers have both their own learning experience of functions, as well as their own teaching experience, leading to the question of what level of student knowledge teachers have related to teaching functions. In this way thirteen mathematics teachers participated in this current research. The research investigated student knowledge determined to be part of pedagogical content knowledge (PCK) by Shulman (1986) and accepted by a variety of researchers as a component of PCK. The teachers participating in the study may be counted as successful in knowing the preliminary knowledge of students and the difficulties and misconceptions experienced by students; however, they were identified to be deficient in terms of knowing how to correct the errors and misconceptions experienced. It also appeared that school type and experience were not important factors. These results have significant implications for developing teaching practices and professional development of teachers.