How to Show a Girl that You Like Her only as a Friend?

[Michael D]

I'm a freshman in high school. There's this girl that I have one class with. She sits to my left, and she talks to me all the time. She asks me about history class mostly, because it's my strongest subject. Whenever we do group work, she slides her desk to mine without even asking me if I want to work with her. I don't mind working with her, but I starting suspecting something about her. One day last week, there was a visiting student at our school, and the student took her seat. She was moved to two seats behind me. The person directly behind me tapped me on the back. I turned around, and she (the girl that usually sits to my left) said, "I miss you." I didn't know what to say, so I just smiled and waved. Later that day, I went on Twitter. We follow each other, so I saw a recent tweet from her to the person who sits behind me that said, "and all Michael did was wave #pissed". I investigated her page a little more, and found more reasons to think she likes me. Now I'm worried that I upset her unintentionally. I don't like her as anything more than a friend, and I have a feeling she has a huge crush on me. I happen to have a crush on another girl, but that's a different story. So how do I show this girl I only like her as a friend? How can I get her to stop liking me? Other things? Thanks.