Stance-taking and stance-support are two discourse behaviours that define the expository/argumentative essay genre, the mastery of which is the key to academic success in higher education. The aim of this study is to discover the extent to which a group of high school students from a non-native English-speaking background are capable of engaging in stance assertion and stance-support behaviours in argument in an online forum, and their repertoire of lexico-grammatical resources for performing these behaviours. Analysis of their online postings revealed that not only do most postings project a stance, a considerable number feature stance-support strategies ranging from assertion of personal belief to rebuttal of anticipated opposing views. Lexico-grammatical structures observed include appropriate forms for realising expression of attitudinal meaning, hypothesising outcomes and other discourse acts in arguing support. These findings are discussed for their implications for the teaching of expository writing in school.