During graduate school, I worked on the largest clinical trial for adolescent depression to date: the Treatment for Adolescents with Depression Study (TADS). Northwestern was one of 13 sites in this clinical trial. 439 adolescents were randomized to receive fluoxetine (Prozac), CBT, the combination of CBT+fluoxetine, or a pill placebo. What we found is that the combo treatment was more efficient in reducing symptoms of depression and suicidality over the short-term (12-weeks). CBT alone caught up to medication and combo at 36 weeks, but it is important to remember that 36 weeks is a long time in the life of a teenager!
If there is functional impairment (meaning the symptoms are getting in the way of the teen's life, such as in school or socially) then it may be important to speed up time to improvement.
Interesting research with animals suggests that extra serotonin may help us learn better. CBT takes a lot of work and some of our mental habits may be very strong. My clinical observation is that medication can help loosen up some of the negative thinking, so that it is a little bit easier to try having the thought "I can do my best to approach this problem" as opposed to the old habit of "I can't do anything right."
Medication isn't necessarily right for everyone and particularly in mild cases it may not be necessary. On the other hand, it can be a very useful tool on the road to wellness.
Mark Reinecke, PhD was my graduate school advisor and the Principal Investigator at the Northwestern cite. You can read more about him here. TADS was funded by the National Institutes of Mental Health (NIMH). NIMH has resources regarding adolescent depression here.
Here are a few of the TADS papers if you are interested in doing some background reading: