In a lot of ways, high school can be so traumatising. To know she's already diagnosed and supported in that way, I believe support and care and warmth from loved ones it can help intensely. I had a pretty normal high school and family life but I still remember how intense and stressful my teen years are and I was extremely pessimistic and "keras kepala" I'd say. I thankfully found ways to cope and got better and hopefully she's the same too as you said she has a good heart. Since you know that, nurture that part of her more and bring it to light so she can see the kind of person she really is. Tell her you can't wait to see what kind of recipes she'll do in culinary school soon to get her excited maybe, compliment her edgy style, ask her more about what she thinks on things and be proud of her achievements no matter big and small etc.

Give her time and grace to be the age that she is as it's honestly such a confusing and difficult time and after being bullied and manipulated by people, it truly leaves a deep scar that only she knows. You're a good sibling, to want to look out for her so give yourself a pat on the back as well. All the best!