It seems that actual software tools utilizing Computer-Mediated Communication mechanisms -like messaging or chat systems- do not help young students to learn good grammar skills. On the contrary, these tools can be detrimental for the acquisition of these skills. On the other hand, it is difficult to design and build software tools for supporting the acquisition of language skills, especially if these tools must support a group of students working in a collaborative way. In this paper we present the design of a Collaborative Learning activity and the corresponding mobile software tool developed to support teaching grammar to primary education Chilean students. Some mechanisms were designed to incorporate Positive Interdependencies in the software tool. The tool was also intended to support Individual Accountability for each member of the group. The developed software tool contains two main interfaces, one for individual work and one for supporting face-to-face group work interactions. The tool is intended to simplify the teacher's task in terms of activity creation and monitoring: the tool automatically corrects students’ assignments and it also provides statistical reports on students’ performance both currently and in its evolution in time. Experimentation was done with 32 seventh grade students (12-13 years old) at a public school in our country. Over 70% of the students thought the activity improved their Spanish grammar, and over 86% of the students found the software tool was very easy to use.