The impacts of the COVID-19 pandemic on physical and mental health-related behaviors among children and adolescents are likely to be profound and long-lasting. This study aimed to investigate the changes in lifestyle and social support and their associations with negative impacts due to the pandemic. A classroom survey using stratified random sampling and structured questionnaire was conducted among Hong Kong primary and secondary school students. The paper-and-pen survey, administered by well-trained research assistants, was completed by 2863 participants aged 9–17 years old (M = 12.6, SD = 1.3) at a brief school reopening six months after the outbreak of the COVID-19 pandemic. About 48% and 37% of the participants stated that they paid increased attention to physical and mental health, respectively. About 20% to 40% stated that they found more support from their friends and family members; only a small percentage reported decreased social support. Around 25% to 50% spent more time to rest, relax, and exercise. The aforementioned changes varied among genders, education groups, and socio-economic status. In general, higher perceived vulnerability, feeling more stressed, apprehensive, and helpless were associated with more reported positive lifestyle changes, including more social/family support, increased mental health awareness, and a positive lifestyle. These positive changes serve as important cushions against the negative impacts of COVID-19.