The aim of this study is to identify nominalized verbal processes and analyze their functions in the written production of secondary school students. These nominalizations are examined in clauses viewed as messages, which are organized according to the Theme and Rheme system. The research is based on systemic-functional theory (Halliday, 1994, Halliday e Matthiessen, 2004) for the study of grammatical metaphor through the identification of metaphorical constructions which present lexico-grammatical variation in the expression of meaning, being therefore considered as more complex than congruent forms. These transformations within linguistic structures start to appear in early adolescence, varying according to new contexts that require specific uses of the language. In order to study the use of nominalized forms in school writing, fifty argumentative essays in Portuguese were collected among 12th grade students from public and private secondary schools. Research participants also answered a socioeducational questionnaire that generated quantitative data so that social and/or individual features could be associated with students textual production. Qualitative analysis led into the identification of nominalizations (Basilio, 2003) with different discursive functions. Concerning the thematic structure, it was observed that nominalizations establish logical relations and help develop the flow of information in texts. A comparison between texts pointed out variation in metaphorical language use in different school contexts. These results indicate that it is important to make students and teachers aware of the textual and discursive effects that some linguistic mechanisms can produce, and based on this knowledge possibly avoid the production of texts that are not adequate to their intended purpose.