This study investigates collaborative attitudes and practices in mainstream Italian schools. Research questions are the following: what kind of teachers' and students' collaboration occurs in inclusive settings? To what extent did significant differences exist among general education teachers and support education teachers? To what extent did training lead to deep-level collaboration practices? Two connected phases compose the research process. The first one is an exploratory phase, subdivided into two stages: four classes of a secondary school were observed through two different structured checklists aimed at obtaining a general exploratory view about teaching practices. In the second stage a survey on attitudes and cultures about collaboration has been implemented and 691 teachers were asked to express their viewpoints (on an ideal and real plane) regarding the co-teaching dimensions. In the second phase, a group of teachers (20) was selected to be trained on co-planning, co-instructing and co-assessing dimensions of co-teaching (Murawski, 2003) and 2 pairs of teachers (control and target group) were video observed in their daily practices in the classroom. Data showed a co-teaching paradox, i.e., the contradiction which often exists between what a teacher believes is important at an ideal level, and what is actually deemed to be important on a plane of reality. Finally, teacher training programs have a responsibility for preparing GETs and STs for collaboration with a focus on strategies to reduce the gap between the ideal and the real situations, starting by reinforcing the ideal vision and supporting the identification of a useful repertoire of collaborative good practices for all in-service teachers.