Aim/Purpose: This paper addresses the effectiveness of flipped learning and teaching as a didactic innovation in math instruction. We are interested in comparing traditional and flipped learning and teaching in terms of acquired knowledge and retention and students’ perceptions of flipped learning and teaching. Background: Traditional lessons, in which frontal instruction prevails, cannot sufficiently address all students, especially in more complex subjects where it is necessary to follow the didactic principle of learning differentiation and individualization. Flipped learning and teaching is a didactic innovation with a high potential for implementing the said didactic principle, as it can be adapted to the students’ needs to a greater extent. There is no single mode of implementation for flipped learning and teaching, which means that the effects depend largely on the specific learning activities, resulting in the fact that previous research does not report conclusive results. Thus, it is important to continue to examine this innovation to provide a better and more detailed understanding of it. Methodology: We present a study in which 13-year-old primary school students took part in a pedagogical experiment in mathematics instruction. In the control group (n = 26), lessons were taught in the traditional way, while in the experimental group (n = 26), lessons were taught according to the principle of flipped learning and teaching. After the experiment, the same posttest was administered to both groups to assess the students’ knowledge of the subject matter after the treatment. Another posttest was administered after three months to determine whether the knowledge acquired through flipped learning and teaching was permanent. All three tests consisted of 13 tasks, with the first 6 tasks relating to the perimeter of polygons and the second 6 tasks relating to the area of polygons. The last task focused on the perimeter and area of polygons. A short survey was also conducted to find out how the students in the experimental group perceived this didactic innovation in terms of motivation to learn, interest in the subject matter, level of knowledge acquired, and so forth. Contribution: Our research on flipped learning and teaching focusing on primary schools is significant as previous research on the topic has often been conducted on a sample of high school students and even more often on university students. Our particular contribution is the fact that we tested not only the students’ immediate knowledge after the intervention but also the retention of knowledge after a period of three months, which provides an additional perspective on the effectiveness of flipped learning and teaching. Findings: With this research, we have answered three research questions. First, we found that there are no statistically significant differences between the two groups in terms of acquired knowledge. Thus, we found that both the traditional approach and the flipped learning and teaching approach were equally successful in transferring knowledge to the students. In addition, we found that there were differences between the two groups in terms of retention of knowledge. The group taught with the flipped learning and teaching approach showed higher levels of knowledge retention than the group taught with the traditional approach. Finally, it was found that the students were quite satisfied with this approach and would like to see such innovations in the future. Recommendations for Practitioners: The results suggest better knowledge retention when using flipped teaching and learning, so based on our findings, we recommend teachers try this innovation in their classrooms. However, all innovation should be implemented with thorough consideration and gradually; thus, we believe that training courses on flipped learning and teaching should be organized for teachers to learn about this innovation, find out about its effectiveness, and reflect on how they can incorporate it into their own practice. Recommendation for Researchers: We recommend that research in the future focus more on primary school students, with particular attention to experimental design. We suggest that researchers focus on investigating the contribution of the different learning activities with the flipped instructional design to the overall effectiveness of the innovation. Impact on Society: The results of our research thus represent an important contribution to the field of pedagogy and general didactics at primary and lower secondary levels. Based on our findings on knowledge retention in the experimental group, we consider flipped learning and teaching to be an effective innovation that could contribute to a higher quality of teaching and, thus, to better student knowledge. Future Research: Future research would be important to determine which factor ensures a higher level of knowledge retention in a flipped learning and teaching approach than in a traditional learning and teaching approach. It would also be important to determine the effects of flipped learning and teaching in other subject matters in the mathematics classroom, in other age groups of students, and in other subjects in primary school.