Every year the Bebras challenge proposes small tasks to students, based on CS concepts. In Italy, in 2021 for the first time, it was possible to choose whether to participate in the challenge individually or in teams of two students. The team size was expected to affect the performance of students; in particular working in pairs was expected to increase the probability of solving the tasks correctly. We carried out an observational study on the results of the 2021 Bebras challenge in Italy, aiming at investigating and measuring the effects of team size on the performance. The findings confirm that working in pairs generally improves the team performance, but the impact is much smaller than expected. We observed that the positive effect of collaboration is greater with younger pupils and somewhat decreases when age increases. We identified and discussed the features of tasks where the impact was more relevant, and where this trend was more evident. We also propose some hypotheses, to analyze in future qualitative studies, to interpret the results.