Freedom Fighter....at 15 years old you can afford NOT to eat so clean. Pick a weight gainer that has a buttload of calories. At this point you should care more about calories than protein. Yes, you need protein too - about 150 grams per day at this point until you add more weight. Add creatine to your diet - I can get it for you for $10/400 grams - mix it with one scoop of the powdered gatorade. Don't believe the hype, you can take it if you are under 18. And it will NOT make you cramp up, but you do need to drink about a gallon of water a day anyways. Add pototoes to your diet. They are a good filler and they are cheap. Cover them in butter, cheese, ham, anything you like.
At your age the key thing to look at is calories. I'll bet you are not taking in near enough. Why don't you post up your daily diet for us to look at. I graduate from high school at 6-1 and 120 pounds. It took me years to add weight. I finally figured it out - my metabolism was too fast for what I was eating. I had to eat at least 6 times/day and triple my calorie intake.
At 36 years old, I now stand 6-1 and 220 pounds, 34 inch waist and about 11% body fat.