I know when you want to break out of the friendzone first step is to cut off all contact. But how do I do that with a girl I go to school with and see every day?

We have been close friends since the day we met. I see her everyday and in class we are frequently talking on a msn like app (I'm on an informatic professional course, so our class is almost always in front of a computer). I am better than she so she usually uses this app to ask my opinion on whatever we are doing and answer her doubts before calling the teacher. But she also uses this to talk about all sorts of stuff. Also, we see each other dayly but she still texts me almost every day.

I know I'm in the friend zone because I am the guy she called when she had a fight with her best friend, flaked on me and more than once she mentioned guys she has a crush on.

My question is: if I am with her everyday how do I set the line on when to talk and when to ignore her? I can't exactly walk past her and ignore her. So far I started only answering msn conversations that are work related, and in a professional tone. Other conversations I just ignore or anwer with yes, no, and other short sentences. On text, I also reply short, don't answer everytime and wait a long time before answering.

But what do I do when I am with her? I've been trying to keep the conversation on a civil, yet distant level, letting her talk and answering short. But it's not like I can avoid her entirely.

BTW, started yesterday and she imediately noticed and asked why I was acting so cold, and today asked again what is going on. I asume that is a good sign. And how do I answer the "why are you cold" question?