Students who have to retake courses at university are often not only low achieving, but also unmotivated and lacking in self-confidence. In this study, we present the first report of a teaching strategy based on the implementation of the flipped classroom model, team-based learning, and frequent testing strategies in groups of students retaking a subject. A sample of seven groups of an average of 68 students followed the new teaching approach. The groups are distributed across four subjects and three semesters. By comparing the average performance across groups that apply different teaching strategies -- traditional versus innovative -- we find a significant increase in the academic performance of the students following the new approach.