Most of opening is not so much what you say but how you say it. You can really say anything to them if you say it the right way. Make a funny comment about the professor. Run an opinion opener relevant to the class or the environment.
But like I said, it's not so much what you say, but how you say it, so rather than regurgitate shit that's already out there check this link out:
steve-p-on-approaching-vt27623.html?highlight=
You wanna get to the point where you do this shit naturally. Best way to get there is approach and open everyone. Hot women, old women, bus drivers, shit even your mailman. It will get you to be so naturally sociable, that opening isn't a matter of how you do it, but what you say to get the outcome you are looking for (ie pickup).