I'll start off by saying I'm 16, and this girl is the same age.
So I got her cell number (via opinion opener), and we'd text, but her replies would generally be delayed, or wouldn't come at all. She wants me to come over tomorrow during the lunch break to eat at her house (her friend will be there also, so there wasn't much of a sexual innuendo there).
Tonight, I joked around on her facebook wall, she answered, i replied. This happened a few times with the messages, then I left a wall post, and she didn't respond. Then I left an inbox message, again she hasn't responded. But I KNOW she is online because she's answering other people's wall posts.
What's going on? What should I do? What should I do tomorrow at lunch? How should I act?
Tips and advice. Needed. Now.

Ah yes, this is a favourite "hard to get" routine ran by girls in their teens. Some girls don't grow out of it until they're in their mid twenties. I think the best thing to do is delay your replies to her messages by almost as long as she delays you. And make sure your next conversation is in person not online or by phone.

Act cool and collected, don't be needy, and talk to her friends in person. Then next time you're talking online, just ignore her when she answers a question. This should spike her interest and start getting you some action.

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Let me start off by saying I think you're too young and not mature enough to be on this site.

This girl is not into you; you're coming on too strongly and probably creeping her out.

Move on and next time, do NOT EVER use facebook as a way of initiating contact, arranging a date, or trying to build rapport.

You will not be successful.