The goal of this clinical trial is to learn if the drug semaglutide changes markers of disease risk as it relates to weight in children ages 12-15 years old who are obese (class 2 or 3). The main questions it aims to answer are: * How do the rate of weight loss, body mass index (BMI), body composition, heart structure and function, and exercise ability interact with one another in the study population at enrollment? * How do risk markers of disease change over the study in the study participants who are given semaglutides to help with weight loss? * Are there differences in the above factors between males and females and are there key factors to help improve the outcomes? Participants will be given semaglutide for this study. During the course of the study, participants will: * have two cardiac MRI scans OR two cardiac echocardiograms (one before starting semaglutide and one around 12 months after taking the drug) * have body composition and fitness levels assessed twice (before semaglutide and around 12 months after taking it) and have urine specific gravity (USG) measured * have extra blood drawn when labs their doctor orders are already being drawn (once at the beginning of the study, once around 6 months after enrollment, and once at the end of the study) * have follow up visits with the study doctor * be asked to take a pregnancy test if they are female and have started menstruation