If your cell phone is on your card, let her know that it is expensive to receive calls on it and should only call when it relates to class - like missing a class.
Set boundries without being obvious that you are on to her feelings. Try to avoid embarrassing her as well as making her feel slighted.
It's tricky, but it can be done.
It's all about setting up the rules.
Funny thing - when I was studying hit medicine I got myself in the study group with some older nurses- they were at about my speed as far as ability to pick stuff up, so I wanted to work with them. Well one of them - one that was MUCH older than me, misinterpreted my generally friendly nature and developed a severe crush on me. I made it clear that I was otherwise engaged - indirectly, but she became spiteful nontheless. You really can't judge how someone will respond, especially when they are developing fantasies in thier own head.
So be very, very careful. And be thankful that it's not a mean spirited elderly nurse!
"Never interrupt your enemy when they are making a mistake."
--- Napoleon
"MonkeySlap is a brutal b@stard." -- SevenStar
"Forgive them Lord, they know not what MS2 can do." -- MasterKiller
"You're not gonna win a debate (or a fight) with MST. Resistance is futile." - Seven Star