Introduction: During lockdown, a student is influenced by a combination of external factors including family environment, increased social and emotional stress, higher educational and computer load, and higher responsibility for doing homework, which inevitably changes his psychoemotional sphere and coping behavior. In order to study characteristics of psychoemotional and behavioral reactions of schoolchildren before and during 2020 COVID-19 lockdown, we conducted a questionnaire-based survey of 1,160 fifth to eleventh graders aged 11–18 in the city of Chelyabinsk. Results: We established a high level of reactive anxiety in 7.9 % and 9.9 % and a low level in 63.4 % and 59.4 % of schoolchildren before and during lockdown, respectively. A high level of personal anxiety was observed in 33.6 % and 42.6 % of children and a low level – in 19.8 % and 16.8 % of schoolchildren before and during lockdown, respectively. Higher levels of reactive and personal anxiety were more prevalent in girls than in boys. We noted a high and increased levels of aggression in 78.6 % and 80.5 % and very high and high levels of neuroticism in 44.6 % and 44.4 % of the respondents before and during school closure, respectively. A high level of emotional excitability and neurotization was observed in one third of the surveyed schoolchildren. The number of respondents with a high level of neurotization and emotional excitability rose by 7.5 % and 4.0 % during lockdown, respectively. The proportion of successful distance learning schoolchildren increased by 16.8 %. Conclusions: Lockdown contributed to the increase in the levels of anxiety, aggressiveness, and neuroticism in school-age adolescents. The number of respondents complaining of sleep disturbance, visual impairment and headaches became 2.7, 1.1, and 1.1 times higher during lockdown. We revealed significant differences in sleep disorders, headaches, dizziness, complaints of vision impairment, fatigue, and irritability between successful and unsuccessful schoolchildren (p < 0.05).