Retrieving information from memory can--under many circumstances--strengthen one's memory of the retrieved information itself. The strategic use of retrieval to enhance memory and help long-term retention is known as retrieval practice. However, it is unclear whether its effect also holds true in the case of learning mathematics. This research is an exploratory case study, where we test a method-- using test-enhanced learning as formative assessment--in a high-needs vocational school. Since the literature on how this method works in classroom settings in mathematics is limited, the purpose of our case study was to get a better view and to lay the groundwork for further experiments. The experimental group and one of the control groups were from the vocational school, while the other control group was from an elite grammar school. On the final test members of the experimental group outscored their schoolmates and reached statistically the same scores as the control group from the elite grammar school. By means of the method, we were able to reduce the performance gap in long-term retention between students from the elite grammar school and students from the urban high-needs school.