The sample of respondents for this research consisted of 44 respondents aged 16 to 19 from the territory of the Municipality of Ugljevik, or pupils of the "Mihailo Petrović Alas" secondary school. The aim of this paper was to analyse the body weight and height of pupils of the third and fourth grade of secondary school, and determine the similarities and differences in the body mass of pupils of different ages. The gender sample of the respondents was unbalanced, hence the obtained indicators, i.e. the values were significantly higher for the pupils of the 3rd grade dominated by boys. For the purpose of measuring the weight and height of pupils, two anthropometric devices were used: scale and anthropometer. Differences between height, weight and body mass index of 3rd and 4th grade pupils of secondary school were determined by the T-test. The results showed that statistically there is a significant difference between the weight, height, and hence the body mass index of the respondents, all due to the unrepresentative sample dominated by boys in one group of the respondents.