During the last decade there has been on-going discussions about students' declining interest and low achievement in science. One of the reasons suggested for this decline is that teachers and students have different frames of reference, whereby teachers sometimes communicate science in the classroom in a way that is not accessible to the students. There is a lack of research investigating the effects of coteaching with senior students in science in upper secondary schools. To improve teaching and to narrow the gap between teachers' and students' different frames of references, this study investigates how an experienced chemistry teacher gains and refines her pedagogical content knowledge (PCK) by cooperating with two grade 12 students (age 18) as coteachers. The teacher and the two coteachers coplanned, cotaught and coevaluated lessons in chemical bonding in a grade 10 upper secondary class. Findings indicate that the coteachers contributed with their own learning experiences to help the teacher understand how students perceive difficult concepts. In such way, the coteachers were mediating between the teacher and the students, thus bridging the gap between the teacher and the students' frames of references. The teachers' PCK was refined which in turn lead to improved teaching strategies.