So this chick in class has been eyeing me a bit. We run into each other the other day, have a nice convo, I toss out some innuendo and get her number since we both have to go. Next day I invite her out and she's busy with some group project for school. Whatever.

Next day is class and I don't really give her the time of day. Now I don't know where to progress from here, though. We text a bit here and there but nothing too serious. Now what?

Idk why you ignored her, but its obvious she is (or was) into you. Stop doing little chit chat and ask her to hang again. If she says she can't again, say something about her being lame and no fun. Just pull some cocky funny in class, being kind of an asshole if she always is a no go.

Davidj is right. Ignoring her at this point was not in your best interest. Hopefully it wasn't enough for her to get rid of you in her mind and you can still redeem yourself. Next time you're in class with her pull some Cocky/Funny bits. Remember, you have to, in your mind, have a set and defined goal and aspiration. That is to get this bird out for a night, maybe more, I won't make any assumptions haha! Make her laugh, engage her in conversation, roll your way into some story (fantasy or DHV) give her some good feelings, her mind rollin and her smiling with you and conversing, afterwards attribute that story to her goin' out with you. Remember, keep your goal in sight, don't wear it on your heart, she'll eat you alive, but don't get too caught up in the nuances and forget what you are after stud!

So in an effort not to compound my mistake, what's the proper move? Should I text/call her and try to build rapport / DHV, and then invite her out again? Or just wait til class and go from there

What I would do if I were you is to talk to her and get her laughing again. If you make her feel good by laughing or building attraction it will help you. Next you should talk something up that your going to do this weekend or this week with your friends or build some rapport and tell her about some of your hobbies. This will help get her excited and less chance of flaking and its not a normal "date".
Build either of these day twos then tell her youll call her to join you.

Thats my 2 cents...