The role of physical education is to act in order to increase the manifestation of motor ability indices among pupils. For this reason, the development of joint mobility in young pupils has a particular importance, by its limiting effects on the adjustments and designation of the movement easiness characterizing the ability of an element to move. The purpose of this paper was to make a diagnosis of the range of motion for the main joints, which included measurements of mobility in the scapular-humeral, coxofemoral and talocrural joints, in primary school pupils from Middle School No. 192. To achieve the intended purpose, it was initiated a comparative study conducted between December 2014 and January 2015, during 14 lessons (according to school schedule and curriculum). The subjects were 80 pupils from 1 st to 4 th grades, participants in the physical education and sports lesson. It was created a group of schoolboys and also a group of schoolgirls for each grade of the primary education, in total 4 groups of 10 schoolboys each and 4 groups of 10 schoolgirls each. Results were obtained using tests for the mobility or flexibility assessment, according to Brian Mackenzie, and through the statistical method, we calculated the arithmetic mean. Analysis of the results provides objective information about the level of mobility development in the subjects tested throughout their evolution.