Many learning opportunities of mathematical reasoning in school encourage passive imitative learning procedures (algorithmic reasoning, AR) instead of engaging in more active constructive reasoning processes (e.g., creative mathematical reasoning, CMR). In the present study, we employed a within-subject mathematical intervention in the classroom with pupils in upper secondary schools followed by a test situation during brain imaging with fMRI one week later. We hypothesized that learning mathematical reasoning with the active (CMR) compared to the passive mode (AR) should lead to a CMR-effect, characterized by better performance and higher activity in brain regions related to semantic memory processing one week after learning. Despite controlling for individual differences in cognitive abilities, higher brain activity in key semantic brain regions such as left AG and left IFG was observed on tasks previously learnt with CMR compared to AR. Thus, encouraging pupils to engage in more active constructive processes when learning mathematical reasoning might have beneficial effects on learning and memory.