English language learners (ELLs), especially those outside major cities in North America, are integrated into mainstream classrooms, often without targeted language support. Form-focused instruction (FFI) is an approach to language instruction in which learner attention is focused on language form during meaning-based activities. Although FFI has proved beneficial for the development of linguistic accuracy in a variety of contexts, its effectiveness for high-school ELLs in mainstream classrooms has not been experimentally investigated. In this study, 18 participants enrolled in a secondary social studies course received FFI on two linguistic structures -- passive and simple present -- for 10 to 15 hours over a 15-week period. A time-series research design was used to compare the students' performance on three pre-tests before the intervention with their progress at the midpoint and immediately afterwards. At each test session, students completed six linguistic tasks measuring both receptive (aural and written grammaticality judgement tasks) and productive (oral and written production tasks) knowledge of both structures. The results of the group's overall performance immediately after instruction indicated limited improvement, with significant gains observed on only two of the language measures. The results are discussed in terms of the challenges of attending to language and content simultaneously.