This study examined how students' science-specific argumentative writing skills and understanding of core ideas changed over the course of a school year as they participated in a series of science laboratories designed using the Argument-Driven Inquiry (ADI) instructional model. The ADI model is a student-centered and writing-intensive approach to laboratory instruction. The study was conducted in two middle school and two high school courses offered at a university-affiliated laboratory school located in the southeast USA. The intervention took place over two semesters and consisted of at least eight laboratory activities in each course. Student learning gains were measured using a science content assessment and a science-specific argumentative writing assessment that were administered at the beginning, middle, and end of the school year. Changes in student performance on the two assessments over time indicate that the students' science-specific argumentative writing skills and their understanding of core scientific ideas improved over the course of intervention. Furthermore, students who participated in a greater number of ADI activities demonstrated greater and more consistent improvement in their writing. The implications of the study describe ways to enhance students' science proficiencies during school science laboratories and include recommendations for future research.