Mate, those were definite IOI's you just didn't notice how you're actually getting some game going!!! Nothing beats a good smile to connect to another person, does it not only show that you acknowledge her but subconsciously implying you're a fun chap to hang with which works when framing yourself as a "catch". Another thing, you also had her disarmed by citing an "unintentional" neg that should've also loosened everything up and that's if your target would acknowledge and respond in a good way in which in your case is a "smile". Now tell me that isn't a good thing and I'll castrate myself.

Finally, don't worry, it isn't a missed opportunity; you guys are in the same environment and its much easier to anticipate movements when you're in the same simulated field which in this case is your class. Use that to your advantage and monopolize her, get her attention by the usual "hey!" or just to follow-up with the last experience you both had and say "didn't I say that you should stop stalking me?" or just plainly smile and ask her about anything that you and her share (i.e gym). I wish you all the best mate and don't forget that number close.
Cheers,
Kraken