I would be really, really certain about their reaction, before you say anything to your parents, particularly at your age. People are usually pretty good at judging the way their parents will react to things, but sometimes they're catastrophically wrong.
Damon didn't even come out as an atheist, until after his parents had kicked him out of the house. He was just challenging the school because of their violation of the 1st Amendment, and everyone went ape-shit.
I've seen these things go both ways. I know a few atheists whose parents are young-Earth, Biblical-inerrantist, anti-gay fundamentalists. Yet, when the atheists brought it to their parents' attention, expecting an absolute shit-storm, the parents took the position that if they reacted poorly, it would only drive their child further away from their religion. So, they sat back and waited for their child's heart to be melted by the love of Christ, or some similar nonsense.
I know two others whose parents are the most fuzzy, new-agey, Oprah-inspired, god-is-love sort of believers, who most Christians wouldn't even identify as Christian. Those parents went absolute freaking nuts, that their child could say that their spiritual beliefs are wrong.
Don't get me wrong; it's usually the more liberal theistic parents who take it well, and the more fundamentalist ones who lose it. There are exceptions, though.
I did it exactly the wrong way, myself. I came out of the closet when I was 14 ... about my lack of religious beliefs, not about my sexual-orientation. People don't generally have to come out as straight to their parents. For that matter, on the subject of homosexuality, some people have had their parents take them coming out as gay in stride, but then the parents took it very badly when the same child came out as an atheist.
Anyway, I was raised as a weekly-mass-going, liberal Catholic, although, it never stuck. I have vague memories of not buying what I was being told, from the age of 5 or 6. When I asked my mother how much longer I had to go to Catechism classes, she said that I had to go until I made my confirmation, which is around the age of 18.
I just kind of blurted out that I wasn't going to make my confirmation, since I didn't believe any of that stuff. It just came right out.
I suspect that the only reason it went so smoothly was because of my father. He went to seminary to become a Catholic priest, when he was 18, but then he dropped out and went to a secular university. I suspect that he learned too much and stopped believing, and he was a closet atheist or agnostic or whatever he thought of himself as.
Anyway, I obviously don't know what your situation is like, but it's worth thinking of the worst-case scenario. If your parents would potentially pull financial support from you for college or if you would otherwise be financially harmed, it's best to keep it to yourself until you're out from under their thumbs.
Sure, honesty is generally the best idea, both from a personal and societal perspective, but if the truth is going to cause sufficient harm ... well, we don't have a commandment against lying, do we? Besides, professing atheism isn't likely to get you out of whatever church services that you hate being forced into, assuming that that's the situation.