A Research Study on How Well Semaglutide Helps Children and Teenagers With Excess Body Weight Lose Weight

Inclusion Criteria: * Informed consent of parent(s) or legally acceptable representative (LAR) of participant and child assent, as age-appropriate, obtained before any study-related activities. Study-related activities are any procedures that are carried out as part of the study, including activities to determine suitability for the study. a) The parents or LAR of the child must sign and date the Informed Consent Form (according to local requirements) b) The child must sign and date the Child Assent Form or provide oral assent (according to local requirements) * Age at the time of signing informed consent. a)Group Kids: 6 to less than 12 years. b) Group Teens: 12 to less than 18 years and Tanner stage greater than 1 * Body mass index (BMI), at screening and randomisation, corresponding to a) Group Kids: greater than or equal to 95th percentile. b) Group Teens: greater than or equal to 95th percentile or greater than or equal to 85th percentile with the presence of at least 1 weight-related comorbidity (treated or untreated): hypertension, dyslipidaemia, obstructive sleep apnoea or type 2 diabetes mellitus (T2D) * History of at least one unsuccessful effort to lose sufficient body weight after participation in a structured lifestyle modification programme (diet and exercise counselling) for at least 3 months * Body weight of greater than 45 kilogram (kg) at screening and randomisation * For participants with T2D at screening, the following additional criterion applies: glycated haemoglobin (HbA1c) less than or equal to 10.0 percent (86 millimoles per mole \[mmol/mol\]) as measured by central laboratory at screening Exclusion Criteria: * Treatment with any medication prescribed for the indication of obesity or weight management within 90 days before screening * Previous or planned (during the study period) obesity treatment with surgery or a weight loss device. However, the following are allowed: a) liposuction and/or abdominoplasty, if performed greater than 1 year prior to screening. b) adjustable gastric banding, if the band has been removed greater than 1 year prior to screening. c) intragastric balloon, if the balloon has been removed greater than 1 year prior to screening. d)duodenal-jejunal bypass liner (e.g., nonbarrier), if the sleeve has been removed greater than 1 year prior to screening * Type 1 diabetes mellitus or monogenic diabetes * Participants with endocrine, hypothalamic, or syndromic obesity * For participants with T2D at screening, the following additional key criterion applies: Uncontrolled and potentially unstable diabetic retinopathy or maculopathy. Verified by a fundus examination performed within the past 90 days before screening or in the period between screening and randomisation. Pharmacological pupil-dilation is a requirement unless using a digital fundus photography camera specified for non-dilated examination