Uncontrolled screen time is a worldwide menace to health of the population. Today, the state of neuropsychiatric health of schoolchildren depends on various factors, including screen time, i.e., the time they spend using mobile electronic devices. This study aimed to investigate how different screen time durations affect the said neuropsychiatric health of this population group. In the 2022–2023 academic year, we surveyed 109 Moscow schoolchildren (35 boys and 74 girls) using questionnaires compiled by A.M. Vane (identification of signs of vegetative symptoms) and S.K. Kulakov (identification of internet addiction). The mean age of the participants was 14.9 ± 0.12 years. The children were divided into two groups: those staying within the regulated limit of mobile screen time (group 1, n = 11), and those exceeding that limit (group 2, n = 98). In group 1, the average mobile screen time, as measured for one month, was 110.50 ± 10.00 minutes per day, in group 2 — 345.00 ± 15.00. The average Vane questionnaire scores differed significantly between the groups (p ≤ 0.01): 12.30 ± 1.89 points in group 1 and 22.54 ± 1.16 points in group 2. Signs of vegetative symptoms were registered in 45.9% of group 1 participants and 63.6% of group 2 participants (p ≤ 0.01). The average Kulakov questionnaire scores differed significantly between the groups (p ≤ 0.05): 28.7 ± 1.88 points in group 1 and 37.1 ± 1.09 points in group 2. Schoolchildren who exceed the regulated mobile screen time limit are at risk of developing vegetative disorders and internet addiction.