Oh I definitely went through this,
there's always the urge to make some elaborate excuse up, but what I've found works best is just to basically say "what's it to you?"

It's kind of along the lines of, "Really??? I am ???? Omg what the... Of course I know that! But thanks for pointing it out anyways."

I always find it funny that people as those questions or point it out as if you were completely unaware until they were kind enough to let you know about it, or nosy enough to pry you for more information.
Just remember, it's none of their business unless you're comfortable talking about it. And there's nothing wrong with talking about it, even though it can seem like the scariest thing in the world sometimes. That being said, there's nothing wrong with telling them to butt out.

I was a big fan of sarcasm in high school, but it was also really helpful to just take a deep breath and say "its a looooong story.". People usually get the point!
Just stick with it girl, it does get better... high school sucks for everyone, especially those of us who stick out.

Not sure if this helps, but my daughter went through a lot of teasing last year when her hair loss was at it's worst. I asked her to think about what it would take for someone to do that. As in, how low do you have to be to make fun of someone with a disability? It helped her to see that there was something way worse wrong with them than there is with her. So, if you think people are asking to be mean, consider what must be going on with them to make them stoop that low...

Practice what everyone has shared with you in front of the mirror to see what you're more comfortable with and also add a smile to the exercise.
When someone asks or points out the lack of hair, being comfortable with your self (you're a beautiful person) will make it easier to move forward from the issue.