Being a teen is complicated, and our brains have not yet fully developed more mature ways of understanding the usual temporariness of depression, let alone the fact that you can seek treatment for it.
The best course of action for any family is to be clear and open with talking about depression. About feelings. About the ups and downs of life. And to perhaps speak of successes we have each had in getting past some pretty awful times in our own lives.
Teens experience a lot of pressure. In school. Social pressure. Pressures related to hormones and changes they are going through. Bullying too is very often playing a role.
It is important for teens to know that they are not alone. That they WILL see better times. That feeling this way is normal. And that if they feel it is going too far, that help is close by, and treatment can be quick and temporary and get them over that seemingly insurmountable hump.