Challenges in educational reform: An experiment on active learning in mathematics

•Experiment designed to affect the ability to reason and argue using mathematics.•Used structured pedagogical intervention aimed at secondary school students.•The intervention was implemented with high fidelity and was internally valid.•The control group learned more than the treatment group.

This paper reports the results of an experiment designed to improve secondary school students' ability to reason and argue using mathematics. A structured pedagogical intervention was created to foster a more active role of students in the classroom. The intervention was implemented with high fidelity and was internally valid. Surprisingly, students in the control group learned significantly more than those who received treatment.