Another thing to remember is that behavior does not equal orientation. Thus, your asexual character is welcome to have sex all they like, and they are still going to be asexual. Maybe your ace character is curious about sex, maybe they feel pressured to do it, maybe they're disgusted by the entire thing and can't even think about it without wanting to vomit, maybe they enjoy sex, or anywhere in between.
An asexual person does not feel sexual attraction to anyone. That's it. Asexuality says nothing about a person's sex drive, a person's romantic orientation, a person's behaviors, or a person's background.
As for the behaviors of an asexual person, you should feel free to draw on the experiences of both yourself and fellow aces. And while there is definitely pressure for females in YA novels to fall in love, it's not a requirement! Furthermore, it's possible that your character doesn't even have the word asexual. I didn't until relatively recently, and until that point, I just said that I'd never felt sexual attraction. For all your character might know, she just hasn't found anyone she's interested in in that way yet, even though she would identify as ace if she knew the word.
Focus more on your character's feelings, and you're probably fine.