When I turned 18 I gave my employer an availability form, now a few months later she is scheduleing me to work outside of my availability, can she do this?

Without an enforceable employment contract or CBA that prohibits it, yes. Just because the employer was able to accommodate your availability at the time you were hired, does not mean they have to do it forever.

Maybe they forgot about it? Have you spoken to your supervisor? Maybe someone else can trade with you?

What about if the reason i'm not available is because i'm still in high school?

The reason is legally irrelevant.

So you're saying that technically they can cause me to either drop out of high school or quit?

We are saying that availability forms are not legal documents; that unless you have a legally binding contract or CBA your employer may legally schedule you for whatever hours they need you for, and that if she needs you for hours outside your availability then that's something you need to work out with her because it does not afford either one of you any legal recourse. It's an internal matter only.