Factual writing is a key macrogenre of American K-12 schooling that is also valued in workplace and society. This study examined the genre and register features of two subgenres of factual writing—biography and report—composed by 48 sixth-grade students in a curriculum unit on scientists and science-related careers aimed at developing students’ understanding of the nature of science. These texts were analyzed for a range of schematic, lexical, and grammatical features that instantiate the two target genres. Statistical and descriptive analyses revealed that the students demonstrated a fairly mature control over the schematic and lexical features that realize the purpose of either genre and relied heavily on the grammatical resources characteristic of everyday registers in constructing both genres. Additionally, there was a positive relationship between the students’ genre/register familiarity and the holistic quality of their writing, and the students’ reading proficiency was a significant predictor of their genre familiarity and holistic writing quality, but not their register understanding. These findings suggest that learning the grammatical resources characteristic of academic registers remains a major and potentially daunting task for many adolescents.