Is 900 calories a day bad for a 15 yr old girl 5'5 tall, very active?

Watch weight: 900 calories may not be enough. Weigh yourself regularly to ensure that you are not losing weight. You should weight about 120 pounds. Try to not weight less than 110 pounds. For good health - Have a diet rich in fresh vegetables, fruits, whole grains, low fat milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form. Get HPV vaccine.

Calories: no, that is not enough for your age, wt and activity.