The aim of the study was to assess body composition using bioelectrical impedance analysis (BIA) among high school students (n = 135) in Lublin. The research was conducted at the university’s dietetic counselling center between October 2023 and February 2025. Participants’ age breakdown: 27% (ages 15–16), 63% (17–18), and 10% (19); by gender: 39% male and 61% female. Body composition was assessed using a foot-to-hand BIA analyzer (SECA mBCA515, GmbH & Co. KG, Hamburg, Germany). The parameters evaluated included relative and absolute fat mass, fat-free mass and skeletal muscle mass. In addition, body mass index (BMI), fat mass index (FMI) and fat-free mass index (FFMI) were calculated. Results showed that 41% of participants had a normal BMI (18.5–24.9 kg/m²), 33% were overweight, 9% were obese, and 18% were underweight. Although underweight was more prevalent among girls than boys, more than 23% of males had an FFMI below 15 kg/m², indicating too low lean mass. The average absolute fat mass was higher in females (21 kg) than in males (18 kg), while the mean fat-free mass, expressed as FFMI, was similar in all sexes and averaged approximately 16 kg/m2. Elevated fat mass, defined as an FMI above 8.7 kg/m², was observed in 44% of female participants. Skeletal mass was proportional to total body mass, and was highest among participants with a BMI above the normal range, averaging 21 kg, compared to 19 kg in those with normal BMI and 15 kg in underweight individuals. In conclusion, BIA is a rapid, non-invasive, and effective method for assessing body composition and represents a promising screening tool for identifying body weight abnormalities in adolescents. These findings may be highly valuable in practice for health promotion and for monitoring the nutritional status of individuals.