When children continue a negative behavior even though they receive consequences for it, our job as a parent is to figure out what they are getting out of doing it, that out weighs the consequences.
 
When you say that you have used consequences, has she made restitution?  Has she apologized to the students she has stolen from?  Has she had to give the item back and pay for it also?  Paying for it with kindnesses, or with money would have similar effects.
 
If this has continued for three years, there is definitely something she is getting out of it.  It may be attention, it may be the excitement of wondering if she will get caught, it may be that she just wants “things” and that is the only way she thinks she can get them.
 
Have you had her evaluated by a professional?  Sometimes there is a distorted thinking pattern that can be remedied by therapy or counseling.  
 
Unfortunately this does not appear to be something that will go away on it’s own like a phase.  If she continues, this will become a very serious problem as you know.
 
Consider talking to the school counselor, and if you've already tried that it may be time to take her to a counselor that specializes in children.  
 
Boys Town National Hotline
1-800-448-3000