We report on an extended effort to introduce and evaluate argumentation as a stand-alone component of the middle-school curriculum. The dialogic-focused curriculum continued over two school years and from a research perspective benefitted from the availability of a closely-matched comparison group who participated in a parallel but non-dialogic curriculum. Following previous reports of success of the curriculum in developing both dialogic and individual argument skills, we report on results for a new cohort of middle-school students that focus on gains occurring after the first year, specifically in the use of evidence to support and to weaken claims. After two years of twice-weekly participation, students were more likely to support claims with evidence, relative to a comparison group. They continued to show difficulty, however, in using evidence to weaken claims. In addition, participants were more likely to draw on evidence from their own knowledge when debating peers than when writing an individual essay.