In a longitudinal design, 51 low-achieving adolescents’ development in writing proficiency from Grades 7 to 9 was measured. There were 25 native-Dutch and 26 language-minority students. In addition, the roles of (1) linguistic knowledge, (2) metacognitive knowledge, and (3) linguistic fluency in predicting both the level and development of writing proficiency were assessed. Low-achieving students improved in writing proficiency, the language-minority students more so than the native-Dutch students. Regarding the level of writing proficiency, individual differences between low achieving adolescents could be accounted for by receptive vocabulary, grammatical knowledge, and speed of sentence verification, suggesting that these are important components in low-achieving adolescents’ writing. Regarding development in writing proficiency, grammatical knowledge predicted variation between low-achieving students. Explanations and educational implications of these findings are discussed.