I've been buying a child ticket for months (16, but I wear makeup and have been told I look 18+, esp when I go out with my baby sister). The trick is, have a fake birthday on hand and be able to say it very confidently when they ask. Second, if it's a long route and they ask where you're going and why, have a logical excuse, along with someone you're meeting (eg. I'm coming back from my grandma's and meeting my dad at the station.) The more "childlike" the reason, the more likely the conductor will assume you're telling the truth and move along. Also, a lot of 12-15 year olds honestly look 17+ nowadays so I wouldn't worry too much. When I got a bus at 12 (first time by myself), I didn't specify that I wanted a child ticket and the driver sold me an adult one...
edit: forgot to add, be confident. although younger teens can mumble/be shy, this definitely gives off the impression you're lying. you can also set your medical ID on your phone to say you're 15 and if they ask for proof of ID, show this instead of a real ID (which is pretty legit, as a 15 year old doesn't have a driving licence or anything).