This is extremely important for teens in their puberty and children!
As you see in the picture, the girl in the middle looks the tallest. The reason is because she is not slouching nor forward, nor backwards. By keeping the spine straight you will avoid deformation over time and such health problems as scoliosis which is common among people who have been sitting in a bad posture.
If you already have the hunchback problem, start going to swimming pool immediately. I talk about the correlation between swimming exercises and growing taller in my other post. This way you will be able to straighten your spine, prevent health problems and get in shape. The spine/posture correction will result in you looking taller!