Initiatives by numerous American associations and various European projects constantly remind of the importance of linking science, technology, engineering and mathematics into a holistic approach to teaching and learning, known as the STEM approach. Comprehensive STEM education research mostly focuses on the effects of this approach on students’ performance and their attitudes towards the approach, whereas a significantly smaller number of studies have examined the instructional efficiency of the STEM approach in science teaching by assessing students’ performance, the mental effort invested in solving tasks and their involvement in STEM classes. With the intention to address the mentioned gap, the aim of this study is to determine the instructional efficiency of and students’ involvement in the STEM approach in biology teaching in primary school, compared to the non-STEM approach. The study employed quantitative research methodology and a true experimental design using pretest-posttest experimental and control groups. The results show that the STEM approach in biology teaching increases students’ performance and involvement and also contributes to the reduction of students’ mental effort. Based on the research results, we conclude that biology teachers should apply the STEM approach in presenting scientific concepts, as this contributes both to the quality of the acquired knowledge and its maintenance.