Mid-adolescence is a period of considerable potential growth in the language for academic writing. Yet, to date, few writing studies explore language development during this period and even fewer focus on longitudinal or diverse samples. In this study, we examined the development of language skills for academic writing in a socio-economically diverse sample followed from sixth to seventh grade (n = 124). In each grade, participants wrote summaries of a science text. Subsequently, summaries were scored for writing quality (WQ) and analyzed for productive language skills (lexico-syntactic and discourse features). Participants completed a receptive academic language assessment and a test that measured reading comprehension of the source text. First, we examined if WQ or productive language skills changed over time. Next, we tested if Grade 6 productive and receptive language skills predicted Grade 7 WQ. Results revealed syntactic growth over time. Grade 6 use of connectives and receptive language skills emerged as predictors of Grade 7 WQ.