In L1 grammar teaching, teachers often struggle with the students’ conceptual understanding of the subject matter. Frequently, students do not acquire an in-depth understanding of grammar, and they seem generally incapable of reasoning about grammatical problems. Some scholars have argued that an in-depth understanding of grammar requires making connections between concepts from traditional grammar and underlying metaconcepts from linguistic theory. In the current study, we evaluate an intervention aiming to do this, following up on a previous study that found a significant effect for such an approach in university students of Dutch Language and Literature (d = 0.62). In the current study, 119 Dutch secondary school students’ grammatical reasonings (N=684) were evaluated by language teachers, teacher educators and linguists pre and post intervention using comparative judgement. Results indicate that the intervention significantly boosted the students’ ability to reason grammatically (d = 0.46), and that many students can reason based on linguistic metaconcepts. The study also shows that reasoning based on explicit underlying linguistic metaconcepts and on explicit concepts from traditional grammar is more favored by teachers and (educational) linguists than reasoning without explicit (meta)concepts. However, some students show signs of incomplete acquisition of the metaconcepts. The paper discusses explanations for this incomplete acquisition.