Girl was writing terrible things about her classmate on social website/She

It depends on the country. As the girl is a teenager, most probably she will not be arrested, but she will be taken to  psychologist, who will try to understand her problems and solve them. If she continues this, her parents will be penied and maybe she will be sent to other school. Her classmates should be closer to her, try to be friends and not to remind her about this. She shouldn't be punished.