My younger sister is depressed about school. I feel helpless. What do I do

My sister(17) has been in a depressive rut for about 2 years now… It got so bad that she admitted herself into a mental hospital due to self harm and suicidal ideation for two weeks. She was put on a new medication, it seemed to help, for a while, until she had to go back to school. School is the main root of this depression, she hates it, it feels so pointless and worthless to her. It’s a chore to do work everyday she doesn’t understand when it feels like it will never amount to anything. She is my best friend. Those weeks in the hospital already felt like years… Its made me so paranoid, every time she closes a door behind her, I feel like she could be hurting herself…I don’t know what I would do without her. I don’t know what to do, or say, or how to help.