How to deal with an odd situation

Hey, 17M here,

So I'm in this art class, and I've been in it for 2 weeks. There's this girl that sits behind me, and I noticed that she was throwing glances at me. She was also flirting with me during that period discreetly. Today, before class started and when we were both in the room, she enthusiasticly walked up to me and introduced herself and it was obvious she was about to do something like ask me out. I was tired and had insomnia, so I literally couldn't even look at her and looked at the ground, and said "Oh, hi, nice to meet you.", then walked away. She looked obviously disappointed, though she was still eyeing me afterwards.

Problem is... she's 14. There is quire a bug maturity gap and I think it's weird. She's a nice person, but uh... it appears that I have gotten myself in a situation.

...What can I do next class to send the message that I think she's nice and a great person, but that I simply cannot be with her? I don't want her to think that she's somehow unworthy or unvalued or anything, but I also want to make sure this problem is resolved.

What can I do?