Peer tutoring is a structured process of co-operative learning, supported by social interdependence theory shown to have cognitive and affective level benefits. During a RCT, this study explores teacher perceptions (n=62) of effects of co-operative learning, including implementation issues, using a mixed methods approach during a 16-week peer tutoring intervention in 58 classes (10 secondary/high schools) in England. Data suggested strengthened peer relationships enhanced student learning. Teacher perceptions remained consistent and are congruent with social interdependence theory and research validating the theory. Implications for research, theory, practice and policy are discussed. Research was supported by Educational Endowment Foundation grant: Spring2012-105.