Harry, if you are concerned then go to the GP and say you have concerns about how you have been feeling but dont want to worry your parents with it as you feel it would make YOUR condition worse. Ask the GP whether you have confidentiality in this issue and whether your parents would need to be informed - state that if they do need to be informed, you will not want to discuss how you feel any further.
If you do things this way, then the most info your parents will get is that you have been to the doc. You could tell them you were worried about a friend and wanted advice
For what its worth, Im almost certain that you are old enough to be covered by the confidentiality act. I would consider going and talking to them to discuss things now if you can.
If you do decide not to then please remember we are here to support you as best we can. There are people from all walks of life on this forum who suffer from depression for a variety of reasons so im sure we would at least be able to make you feel more supported than you do at present. You are not alone.