Just be casual, and like season 7445 said, try to say something light and funny for an opener; something that the both of you can relate to or what's going on around you. For instance, make a light joke about something that might have happened during class when you're walking out by one of these girls.
The trick is to keep it natural, and to avoid making her think you are hitting on her(which she will know once you get her phone #).