Providing formative feedback to foster collaboration and improve students' practice has been an emerging topic in CSCL and LA research communities. However, this pedagogical practice could be unrealistic in authentic classrooms, as observing and annotating improvements for every student and group exceeds the teacher's capabilities. In the research area of group work and collaborative learning, current learning analytics solutions have reported accurate computational models to understand collaboration processes, yet evaluating formative collaboration feedback, where the final user is the student, is an under-explored research area. This paper reports an exploratory evaluation to understand the effects a collaboration feedback report through an authentic study conducted in regular classes. Fifty students from a Computer Science undergraduate program participated in the study. We followed an user-centered design approach to define six collaboration aspects that are relevant to students. These aspects were part of initial prototypes for the feedback report. From the exploratory intervention, we did not find effects between students who received the feedback (experimental condition) report and those who did not (control condition). Finally, this paper discusses design implications for further feedback report designs and interventions.