Enhancing understanding is crucial for "all" students. Instructional approaches have been developed to achieve this goal, yet little is known about the kind of support that teachers need for their effective implementation. We compare two support conditions designed to enable teachers to enhance students' understanding of the mathematical topic percentages. 51 teachers and their grade 7 classes participated in a quasi-experimental pre-post-test control group design, comparing three groups. (1) Teachers of the material condition received the curriculum material of the MATILDA teaching unit on percentages, with a comprehensive teacher manual on the main principles of the approach. (2) Teachers in the workshop condition received the same materials and additionally participated in professional development (PD) workshops covering key ideas of the approach. (3) The waiting control group used their regular textbooks. Hierarchical regressions were used to compare the effects on students' learning outcome. The analysis reveals that students in both support conditions achieved significantly higher learning outcomes than the control group, i.e., curriculum materials and teacher manuals seem to provide effective support. No additional effect on student outcomes was found for the workshop condition, but a substantially lower teacher dropout than in the material condition. This finding indicates that PD workshops may not be necessary for highly motivated volunteer teachers but can be completed by a wider range of teachers. The findings can inform implementation projects for educational innovations and inspire further research to determine for which teachers a written PD is sufficient and for which face-to-face workshops are more promising.