Hey Nandita,
Weather its a teenager or adult, upper back pain can be found in students and people having long time desk job. It can be because of muscle stiffness , inflexible back muscles, weak back muscles, any body organ problem, radiation of some cause any where else. etc. So depending on that you need to cure the cause. Try some stretching and some strengthening exercises for your upper back muscles. If still it persists then you need to consult a doctor.