Modern lifestyle and the development of technology have influenced the living habits of almost all people, especially adolescents. Spending free time in a large percentage is reduced to looking at different screens, mostly while sitting, therefore a sedentary lifestyle is extremely common nowadays. High school students additionally spend 6-8 hours sitting in school desks, and all of this can have negative consequences on body posture. The aim of this work was to determine the acute effects of a three-minute exercise program (carried out on a chair) on the upright and sitting posture of adolescents. The research was conducted on a sample of 33 students of the Architectural-Geodetic School in Osijek average age 17.08 ± 0.50 years, of which 25 are male students (average height 177.04 ± 5.73 cm, body weight 72.92 ± 11.52 kg ) and 8 female students (average height 162.62 ±6.82 cm, body weight 61.14 ±10.32 kg). The subjects were initially photographed before the three-minute exercise program on the chair and finally photographed after the exercise program in the same way. The photos of the initial and final measurement were processed in the Apecs program, from which the monitored variables were obtained, namely: head lateroflexion, shoulder height inequality and body lateroflexion in standing and sitting positions frontally (anteriorly) and forward body tilt standing sagittally. Differences in the results of the initial and final measurements, as well as their statistical significance, were verified by the t-test for dependent samples, while the effect size of the applied set of exercises was evaluated by Cohen's index of effect size (ES). The variables were followed by indicators of the posture of the head, neck and upper part of the trunk standing frontally and sagittally and sitting frontally. The obtained results showed improvement in all monitored variables, i.e., there is a visible trend of reduction in the results of the final measurement compared to the initial one. A statistically significant (p<0.05) reduction was present in the variables: head lateroflexion, shoulder height inequality and body lateroflexion in the upright posture (frontal anterior) and in the variables: head lateroflexion and shoulder height inequality in the sitting posture (frontal anterior). In other variables, no statistically significant reduction was demonstrated, but in all variables, a small to moderate effect of the applied exercise set was demonstrated. Considering the obtained results, the proposed three-minute set of exercises on the chair could find a purpose in preventing/improving bad postural patterns at the level of the head, neck and upper back in adolescents in such a way as to be included in the physical and health education class or to encourage introduction of one active break during the school shift. Considering that the exercise program was carried out on a chair, which is characteristic of school teaching, the three-minute set of exercises could also be applied in the classroom before the start of almost any subject.