The class thing works but if your past that point in your life....

Basically anything that breaks the ice to start talking will work if you play it right (picked up a girl in the airport by making a comment about another girl). If the girl is cute but not that hot then use something really dumb and obvious and she will be flattered and take to you (ask where something is thats right behind you or something). If she is really hot then you can either start in on saying something dumb stop yourself and explain you just wanted to talk and generally they'll take to it cause you are being honest or just walk up and start talking to em like you would anyone else. Most of the really hot girls don't get approached because guys are scared to talk to em. Also really hot girls are used to getting truly hit on not talked to and have heard all the lines so they won't work. With them honesty and being confident without being cocky works best. Talk first (even if she's dumb as a brick) then move on after you have her entertained/curious. Oh and you gotta play a little cause girls like to try to figure you out and chase.

If you get the cold shoulder and feel like having some fun then you can tell her its your birthday and she's got to be at least nice to you on your birthday. Girls love birthdays and will actually change and be nice to you a lot of the time and then you got a shot. Just don't hold out to long before you tell em the truth cause if you do they will be pissed (or remember in a year and throw you a surprise party and invite all your friends... bad bad day).