The objective of this research is to observe argumentation in high school student written production from public schools in Rio de Janeiro through the study of lexical elements with modal function, as well as textual sequences with value of judgement. The theoretical framework was based on assumptions from: Systemic-Functional Linguistics (Halliday, 1994), aiming at the analysis of modality (Halliday, 1994; Eggins, 2004; Thompson, 2004); and Appraisal Theory (Martin e White, 2005; Hunston e Thompson, 1999; Stubbs, 1996), especially the subsystem of Judgement, that was used to investigate the assessment that the authors assume and express through text as message. Concepts from argumentation theories (Osakabe, 1999; Possenti, 2001) substantiate the observation of the elements that influence argumentative strength in texts. Research methodology followed a qualitative approach to text analysis. The corpus selected for analysis consists of essays written by high school students, produced during classes of the discipline Textual Production. The analysis of the texts was developed according to two complementary phases: a) observation and description of the lexicogrammatical elements that helped create argumentative strategies intended by students; and b) observation, description and interpretation of argumentative sequences in text progression, on the basis of judgement identified in the essays. The considerations raised after detailed examination of the data show that the selected texts present argumentative strategies related to modal and evaluative elements, such as polarization and the use of textual sequences of judgement. Results also point to a variation in the linearity of argumentative texts and allow for reflection on pedagogical practices in relation to teaching argumentative genres in the school context.