A classmate of mine has a cutting habit and I'm going to expose it to the whole class.
Why would you want to expose their self-harm habit?
She has emotional scars already due to her looks and that's why she cuts herself.
You shouldn't have to keep a secret like that to yourself, but if you expose your classmate's issues to the rest of the class, it will surely hurt her even more. Try talking to the school counselor instead to try to get her some help.
I don't know if that's the right thing to do, I'm just upset.
I get why you are upset, but how would you feel is someone exposed a serious personal issue you had? You might truly harm your classmate even more. Try to be mindful of what she's dealing with and try to get her help in an appropriate manner.