This study discusses the experience of the implementation of a peer-norm in reducing free riding and conflict problems in teamwork activities. This exploratory experiment is run with two cohorts of students in one of the principal Spanish universities. The educational problem is to control for student potential collusion (with clear free riding episodes) in homework projects performed by assigned groups. The peer -norm intends to control for it. The empirical analysis identifies that the success of this norm is associated with the presence of foreign students. Empirically, the presence of foreign students in each cohort brings an educational and cultural background different from the native's one. Their presence in work -groups turns out to be the key factor to reduce potentially the degree of collusion in the overall cohort because they seem to be less concerned by the "social retaliation" of the mates.