Jealousy about a guy friend ?

I'm friends with this guy and we are quite close, and I really like him, and like that we are able to tell each other anything. We have been friends for a little over two years, and he was in my chemistry 11 class this year. People say he likes me more than just a friend because apparently he treats me different than any other girl, but I never really thought of us as anything mote than friends. So there's this girl that sits next yo me in chem class and my friend sits directly in front of me, and she told me that she likes him. He doesn't like her, he makes fun of her, and it's usually very innocent the way he makes fun of her, but to me he always is helpful and nice and funny. She is getting upset at me and cornered me to tell me to "stay away from him because he's just talking to you to get me jealous" I tried to explain that I am friends with him but she won't listen. I don't know what to do. Any suggestions? Thanks :)