*sigh* yeah, no matter how many times you tell them, you're not going to make them believe you.
Many parents go into denial with the less obvious sorts of autism. Mine did. I wasn't told for nearly ten years, and even after that my mom only accepted it if she could say that it "wasn't really a disability" (yeah, ask the US government, the hospital, half a dozen doctors... they'll tell you different).
My mom had this idea that disability means you're completely and totally unable to do anything, that people will cheer for you if you learn to button your shirt but never expect you to do much more than that. She believed that your life was over if you got a disability. When she talked about it, you could hear the horror in her voice. It was like the worst thing that could ever happen to you. When my sister and I used to do things that weren't safe, she used to threaten, "You'll spend the rest of your life in a wheelchair!"
You can see how this clashed with her idea of her "intelligent" daughter who, okay, maybe didn't have a lot of common sense, maybe was strong willed and "didn't want to" play with other kids (hey, she plays with her sister, right?)... maybe spoke a little oddly but hey, look at this huge vocabulary... who got good grades in school, and was so obviously capable of cleaning her room and taking showers and doing chores and controlling her "temper tantrums" that any failure to do the above was simply "strong will"... despite that those "temper tantrums" lasted even through the teen years, even though every time they happened they resulted in getting pinned down or hit or belittled... My mom thought just I needed a proper father to "handle" me... so she married them, never realizing just how rough that "handling" was going to be.
But I'm autistic. In fact, apparently I'm obviously and strongly autistic, though not disabled enough to fit into my mother's disability stereotype. My mom still doesn't seem to believe that this is real, even years after the official diagnosis. It's as though she thinks these are normal traits, and I'm just exaggerating; or else autism isn't "real" unless you can't talk. The same thing she's told me all these years, she keeps telling me: Just "try harder", and you can do anything you want. Your weaknesses aren't real; they're just laziness. She points out that other people cry, too, as though that would make my crying for six hours over a missed appointment "normal" and less headache-inducing.
Point being: You can't talk your parents into believing you are autistic, if they don't want to believe. They can live with the obvious signs for years. My mom isn't ignorant about autism, either; she's an occupational therapist who's worked with special ed, for heaven's sake, and if anyone should know about it, she would (well, she did, and admitted as much to me; but she didn't believe it!).
Parents are capable of unimaginable amounts of denial, and your telling them won't make them understand. It'll just make them angry.
If you need help with something, your best bet is not to mention autism at all, but to be very specific. Rather than say, "My autism is causing problems with my schoolwork," say, "I got a C- in biology this year. I need some help with...." and then the specific skill that is creating the roadblock. They may have gotten this reflex going that "if I hear autism, I'll assume it's not important," so don't mention autism.
This is survival. You've got to do what you've got to do, and if you have to suppress talking about autism to certain people who flip out when they hear the word, then you're going to have to do that. If you can get really specific about what you need, you might be able to sneak some of the required accommodations right under their radar.