Remember boys and girls, bmi is not actually a good measure of body composition or health. Everyone should do at least some strength training: arms legs and core. This will only work if combined with a protein-rich diet of chicken and/or whey. Especially important for girls with a lot of fat on their butt and chest. That extra muscle, beside being healthy, will make your bmi go up even though it's muscle and not fat!
How much you can plank, deadlift, hours you can walk, and so on is much more important than worrying about chub around your waist.