Girl Won't Respond to me...

So I've been chatting with this girl from my school. Just about every day I'll message her, but today/last night she has only replied once, then I'll say something, and she just won't reply. I don't want to sound desperate and keep messaging her, but I also want to chat. She goes on Facebook without messaging me, cause I leave it open and will see that she's online, but she won't say anything to me, but maybe update her status. It has happened before where the notification didn't pop up for her, but that was only once and I usually don't assume that happens. I haven't said anything to offend her, and today the one time she did reply to me she sounded excited and had some smilies. Furthermore, we have plans to hang out sometime, we just haven't set a time. What should I do? FYI I'm a 17 y/o male. Thanks!