You may have developed a curve in your back over time. But the good news is you can use yoga for slouching. It will help reinforce the natural curve in your back and improve your posture.
This is common, especially since girls often grow rapidly at the onset of puberty and end up being several inches taller than the boys! For teenage girls, stooping in the shoulders may also reflect an attempt to conceal growing breasts. As a result many girls grow up with a rounded upper back—what I like to call, "Schlumpasana." It will take time for you to extend into your full height—into all your grace and beauty—and undo a pattern of slouching forward. Practicing yoga poses certainly can help.
Begin to establish some strength in your arms. Practice Adho Mukha Svanasana (Downward Facing Dog) by pushing into your hands, stretching your fingers open, and lengthening through your forearms. Building stability in your arms this way will begin to help you anchor your shoulder blades onto your back.
Stand with your legs hip width apart and interlace your fingers behind your back. Draw your shoulder blades down your back, thereby engaging some of the muscles that serve to anchor your scapula down. You can repeat this anywhere, anytime throughout your day to relieve tension and increase mobility.
Practice Salamba Sarvangasana (Shoulderstand). Be sure in Shoulderstand that you bring your elbows in (the upper arms are meant to be parallel to one another). You may position yourself up on several blankets to be sure that you do not press the back of your neck to the floor. Shoulderstand and Halasana (Plow Pose) will help to yoke your shoulders back and give you much needed elasticity in the rotator cuff muscles around your shoulders. Hold Plow or Shoulderstand for 2 to 3 minutes, eventually building up to 5 minutes. This will bring ample blood flow to this area, irrigating it and making your shoulder girdle more mobile and free!