There is a clear contemporary interest for developing science, technology, engineering, and mathematics (STEM) at schools. Besides, there exist a lot of research that justify the importance of spatial ability to obtain success in STEM subjects. Nevertheless, the spatial ability is relatively ignored in the general practice of teaching and learning in the K-12 setting. The goal of this paper is to analyse the evolution of spatial abilities of students that assist to a STEM course. Additionally, the evolution of their mechanical reasoning is also analysed. The STEM course was designed and implemented for the first time in a 6th grade class (primary school) and a 7th grade class (secondary school) throughout a whole academic year. First, the spatial ability and the mechanical reasoning of the students were assessed with the corresponding prepared pre-tests. Then, after finishing the STEM course, the students were tested with analogous post-tests. An exhaustive analysis of the obtained results is provided in the paper. It is shown that the spatial ability of the students was definitely improved. Furthermore, this improvement was statistically significant. Results also evidenced that the mechanical reasoning of the participants was also improved, although the improvement was not statistically significant. Moreover, this research showed that, in general, obtained results do not depend significantly on the gender of the participants. Finally, results manifested the statistically significant difference of spatial ability between 6th grade and 7th grade students. The difference between grade levels was not as significant in the mechanical reasoning case.