Changing practices in schools is a very complex endeavor. This paper is about new practices we prompted to foster collaboration and critical reasoning in science classrooms: the presentation of pictures representing different perspectives, small group synchronous argumentation, and moderation of synchronous argumentation. A CSCL tool helped in supporting synchronous argumentation through graphical representations of argumentative moves. We checked the viability of these practices in science classrooms. To do so, we investigated whether these practices led to conceptual learning, and undertook interactional analyses to study the behaviors of students and teachers. Thirty-two Grade 8 students participated in a series of activities on the day/night cycle. Learning was measured by the "correctness" of knowledge, the extent to which it was "elaborated", the "mental models" that emerged from the explanations, the "knowledge integration" in explanations, and their "simplicity." We showed that participants could learn the day/night cycle concept, as all measures of learning improved. For some students, it even led to 'conceptual change". However, the specific help provided by teachers during collective argumentation did not yield additional learning. The analysis of protocols of teacher-led collective argumentation indicated that although the teachers' help was needed, some teachers had difficulties monitoring these synchronous discussions. We conclude that the next step of the design-research cycle should be devoted to (a) the development of new tools directed at helping teachers facilitate synchronous collective argumentation, and to (b) activities including teachers, designers, and researchers for elaborating new strategies to use these tools to improve the already positive learning outcomes from synchronous argumentation.