Using the accurate relative pronoun (RP) in a formal writing task presents challenges for writers since they seem to be influenced by forms used in the popular oral variety of French which are far from the linguistic norm (Blanche-Benveniste, 2010). Studies describing the teaching of the relative clause (RC) in the secondary classroom have highlighted the problems encountered by students not only with handling this grammatical object, but also with using their grammatical knowledge in revising their text (Dolz & Schneuwly, 2009). However, to our knowledge, no study has yet been conducted to conceive and test an intervention for teaching RCs in French L1 classes. Based on theoretical and empirical work converging toward the fostering of sustained verbal interactions throughout grammatical and revision instruction, a series of lessons was implemented with 52 grade nine students enrolled in a French course (Montreal, Canada). Pretest and posttest texts were analysed in terms of RC frequency, usage and accuracy. While no difference was found in the general frequency of RCs, results show a significant increase in the use of complex RPs. Students, especially the weaker ones, also make significantly fewer mistakes overall on RPs and also on complex RPs. These results could indicate that certain structures associated with complexity and formal register are used more frequently and more accurately during written production after our intervention. Our results contribute to the ongoing discussion on the complementarity between direct grammar instruction and writing and revision instruction and their positive impact on students' syntactic constructions in texts.