Lunch Date

So today was just weird...
There's a chick in my 2nd Year classes this semester who seems to have taken quite a shining to me.
She's made an effort to sit somewhere near me in most of my classes and managed to get herself included in one of my group presentations for the semester as well.
The first day I met and spoke to her I thought she'd picked up on the blatantly obvious. I'm not so sure this is the case though.
She doesn't strike me as the brightest crayon in the box.
No idea how it came up in conversation but she was telling me how she tries to good by everyone and that she hates when she hears people saying bad things about her, and something about some crazy lady from her 1st year of studies that hates her so much she wants to put a hit out on her.

So I made the comment, yeah Karma is a wonderful concept. Her reply... I'm Christian, I don't believe in Karma. I believe what goes around comes around.
I bit my tongue in order to prevent myself pointing out that that idea was the essence of Karma.

So anyways, she added me as a friend on Facebook and me being me accepted.
I noticed the other day that she was complaining about how much trouble she was having drawing up a Genogram for one of her assignments. They're pretty piss easy to do in my opinion, so I offered to meet up with her earlier today and give her a hand.
She jumped at the chance and offered to buy me a coffee in return... now who am I to refuse that!?!

I met her in the Cafe at uni, and her offer of coffee quickly turned into her offering to buy me lunch as well. This set off a few alarm bells in my head, 5 mins of work does not constitute a free lunch and coffee!
Suddenly it dawned on me... fuck, I don't think she realises that I'm gay and oh fuck! she's trying to turn this into a lunch date!
I politely declined the offer of lunch making up some excuse about having just eaten and looked over her work. I couldn't find any fault with it... she'd downloaded a program after I'd arranged to meet her to do it all for her! So it was quite literally perfect.
I gave her a few pointers on what else she should include in it and was about to leave when she asked me to explain the Micro, Messo and Macro systems theories to her.
By this point I was like, WTF, are you looking for excuses to hang out with me?

Again, I explained what I could using my simple analogies (the ones they spiel off in class are confusing to say the least). She then offered to walk me to my car, I told her I was out the front of the university. She told me she was too (fuck, no chance of escaping out of this one!). So I agreed to walk with her, turns out her car was parked in the complete opposite direction to mine so I was able to escape without any major hassles (thank fuck for small mercies!).

But now I'm left with the fun times of making her understand that I'm not interested... I mean sure, I could've completely misread this entire situation, but I somehow doubt it.
I actually feel a wee bit sorry for the poor girl if what I think is right!

But now I have no idea what to do... How the hell do I get it through to her that I'm gay?