@samm7720,
Well, telling her you like her is not the way to go about it. Which I am sure you realize now.
But also keep in mind that you're only hearing one side of things. Maybe your pal Judy is exaggerating. Because, y'know, teenaged girls never, ever exaggerate.
But let's go with your initial question.
I know you don't want to hear any advice about talking to a teacher, even though they exist (in part) to try to help young people who have problems in school.
So, let's talk about you taking matters into your own hands.
You don't have to be out and out rude, but you can certainly ignore Ash/keep interactions to a minimum.
For example, she asks you if you're free Saturday night.
You say any of these (adapt as necessary):
* Sorry, gotta study.
* Can't, it's family game night.
* No can do, I promised Judy we'd go out for tacos (this doesn't have to mean you fake going out with Judy. You're just 2 friends getting tacos)
* Sorry, I've got baseball/violin/knitting/whatever practice that night.
* I am finally getting around to watching Game of Thrones (or whatever) and I prefer being alone for that.
Short, sweet, with no open backdoor. And these aren't really apologies; they're conversational niceties. Which you can alter or eliminate. E.g. the first one can just be, I can't, gotta study.
Oh, and for the ones where you say you'll be at home, stay home for real. Otherwise, you'll end up in even more of a bind and will have to face
The Wrath of Ash....
(Which sounds like a b-movie).
Are these lies? Why yes, yes they are.
Honesty, when it comes to sparing someone's feelings, isn't always the best policy.
And I still say involving a teacher could turn out to be a good idea.