Formative assessment (FA) is considered a powerful tool to enhance learning. However, there have been few studies addressing how the implementation of FA influences the development of inquiry skills so far. This research intends to determine the efficacy of teaching using FA in the development of students' inquiry skills. A test of inquiry skills was used as the main research instrument to identify the level of the selected inquiry skills before and after a period of teaching with or without FA in the experimental and control group respectively. In the experimental group (n=73), FA was implemented across five subjects: biology, chemistry, physics, mathematics, and informatics, while the control group (n=48) was taught without any external intervention. In both groups, mostly teacher-centred methods of teaching were applied. The research findings confirmed a low initial level of inquiry skills. Teaching using FA showed higher efficacy than teaching without FA, even without a special focus on the inquiry methods. The experimental group achieved significantly better results compared to the control group. In addition, the results confirmed higher efficacy of FA in lower performing students. Gender did not prove as a determining factor in the effective implementation of FA.