I need advice on how to deal with an obsessive person.

I know these questions about obsessive friends are asked to death and for that reason I’ve put off asking this question. However, I’m at my wits end and I’d really appreciate some thoughts on how to move forward with this situation. I’ll be as concise as possible. Please bear with me :-)

There’s a girl I know through unusual circumstances – we both met while unwell. That’s the only thing we have in common.
She’s extremely obsessive and it took me a while to confront her about it as I was worried about upsetting her, etc.

I’ve had energy draining friends before and I’ve managed to walk away from the friendships. This is a little different.

She turns up at a lot of the places I visit with other friends. If I tell her I can’t meet because I’ve plans, she’ll be there or arrive. (We have some mutual friends on facebook as we’re from the same town so she sometimes gets her information re our plans there.)
I started a new course in September, which she knew I was doing. When I went to enrol, she was there to enrol on the same course. It’s all becoming a bit single white female for me.

It’s a very small class so I’ve tried very hard to be diplomatic. I cannot bear drama either.
I can get 10–15 calls per day from this person.
I’ve asked her not to call me like this as I find it invasive and she reacts in an overly confrontational manner – usually shouting at me.
It’s all become too much. I feel I’m doing the right things, but as our next semester is starting up next week, I’m very worried about having to ride out the storm. She can be quite aggressive and unpredictable and I’m looking for advice about how to stay strong through that.
We’re both in our mid twenties. I’d imagine this all sounds very high school.