Beyond mechanics and spelling conventions, academic writing requires progressive mastery of advanced language forms and functions. Pedagogically useful tools to assess such language features in adolescents’ writing, however, are not yet available. This study examines language predictors of writing quality in 51 persuasive essays produced by high school students attending a linguistically and ethnically diverse inner-city school in the Northeastern United States. Essays were scored for writing quality by a group of teachers, transcribed and analyzed to generate automated lexical and grammatical measures, and coded for discourse-level elements by researchers who were blind to essays’ writing quality scores. Regression analyses revealed that beyond the contribution of length and lexico-grammatical intricacy, the frequency of organizational markers and one particular type of epistemic stance marker (i.e., epistemic hedges) significantly predicted persuasive essays’ writing quality. Findings shed light on discourse elements relevant for the design of pedagogically informative assessment tools.