I don't really like deception, but if you absolutely feel you can't tell your parents, it may be needed. Depression comes with physical symptoms, like headaches or sleeping problems. Tell your parents you are suffering badly with these and need to see your doctor. When you get there tell your doctor everything you've told us. S/he can help.

As a teenager I had BAD anxiety and depression. I felt I couldn't tell my parents. I didn't have a valid reason like you do, but I just couldn't. I felt like they would see me and themselves as failures. I was so wrong. I waited until I had something like a breakdown. My mom caught me trying to hurt/kill myself (she was supposed to be at work, but had an urge to come home and check on me). As a result I was taken by ambulance to the ER and EVERYONE in town found out about my problems. My parents were concerned and supportive. They just wanted me to be well. I wish I had just told them.