It's a Japan thing. Apparently, it's from anime. Senpai means someone you're suppose to look up to. (in this case a senior or a teacher if you're thinking of that chick from Misao who liked her teacher.) All in all 'Notice Me Senpai' means you want someone older than you to like you.
An expression of disgust or annoyance generally at the wanton stupidity or crassness of another, a facetious request for the Last Rites, indicating one has thrown his or her hand in, and given up altogether.