I can't gain weight..ㅠㅠ

If a 4th grader weighs 23kg, they are considered thin, and I always hear "Eat more." According to the instructor, it's because they attend six different sports academies. Types of academies: Flying Disc, Floorball, Soccer, Taekwondo, Regular Jump Rope Class, Special Jump Rope Class. Study academies: English. If anyone knows how to gain weight, please comment.