Learning to write in middle school requires the expansion of sentence-level and discourse-level language skills. In this study, we investigated later language development in the writing of a cross-sectional sample of 235 upper elementary and middle school students (grades 4-8) by examining the use of (1) lexico-grammatical forms that support precise and concise academic writing and (2) paragraph-level structures for organizing written discourse, known as micro-genres. Writing studies typically elicit and analyze long compositions, instead the present study employed two brief writing tasks that allowed for the evaluation of language skills while minimizing the influence of topic knowledge and other non-linguistic factors. Results of structural equation modeling revealed that the two facets of language proficiency studied--lexico-grammatical skills and skill in producing paragraph-level structures (micro-genres)--represented distinguishable dimensions of productive language skill in this sample. On both dimensions, older writers (grades 6-8) demonstrated greater skill than 4th and 5th graders. These findings, which provide an initial proof of concept for the use of short writing tasks to study language skills that support academic writing, are discussed in relation to writing theory and pedagogy.