Debates have been used to develop critical thinking within teaching environments. Many learning activities are configured as working groups, which use debates to make decisions. Nevertheless, in a classroom debate, only a few students can participate; large work groups are similarly limited. Whilst the use of web tools would appear to offer a convenient solution, none of those currently available provides an automated system for organizing contributions into a logical structure, or for making decisions. To address this problem, this paper describes a new tool for managing and structuring debates over the Internet, and presents the results of a series of trials in an educational context. The tool enables users to post opinions and proposals, and to make multiple group decisions. The main advantages are that it does not require a moderator, and all contributions are automatically arranged into an intuitive structure. Thus, it enabled large groups to carry out bigger projects. Empirical results showed that it also encouraged the involvement of all the students in debates and allowed the participation of each student to be evaluated. The tool demonstrated its advantages over traditional oral debates and, as far as we are aware, it incorporates features not found in any other comparable web tool.