I take my son's depression seriously because I know now that if he wanted to kill himself he could. And, it is a lot "easier" than we probably thought it could be. Children do not have any real concept of just how easy it is to kill themselves - until it is too late of course.
Having said that, I think it must be very hard for a GP to tell the parents without breaking any confidentialities.
I left home at 16, I was earning a wage and paying rent. I think it may even depend on the age of the GP and how seriously they may or may not take teenage suicidal tendancies. I firmly believe that if a child is living at home, then the parents ought to be informed. It would be much harder if the child has left home because after all they are not exactly a child any more if they are supporting themselves.
In answer to one of your questions, yes I take my son's depression seriously because of his AS as AS teenagers have a high suicide rate due to their not wanting to be "different" to their peers.
AT 9, it was very easy for me to go into the GP room with him. At 13 it is not. If he does not want me in there, I have to leave. In the same way that if he gets upset with the GP and leaves their room, they ask me to leave as they cannot discuss a child who is not actually present in the room.
So, all in all, it is a no win situation for a parent.