Hi Angel,
People cut themselves for a lot of different reasons and they rarely mean to actually truly hurt themselves. However, they often get carried away and end up severly hurting themselves.
One thing that may help is called, "Cognitive Behavioral Therapy."
This is a kind of therapy where the believe is that: we learn to react to stress a certain way and so everytime there is stress there is an automatic need to react the same way. In the therapy sessions you learn to react differently to stress.
For instance a person may fail a test. The test failure makes him think. "I'm stupid" Thinking "I'm stupid" then makes him think "Nobody loves me." Thinking that then makes him think "This hurts" then cutting himself relieves the mental pain.
Cognitive Behavioral therapy teaches the brain to think like this:
Person fails a test. The test failure makes him think, "I didn't study the right material"..."The test was very hard"..."I need to talk to other people in the class and get some help for the next test"
This helps people take the negative focus of of themselves and begin problem solving.
Your girlfriend needs real help. She is clearly not coping. You were right to call her father, but perhaps next time you should get some help in approaching him. It may be useful to talk to one of your school counselors about how to get some help.