Hi and hellos is all good openers, very clasic however you need to have something strong, interesting eye catching convos to back up that hi/hellos. You need to catch her attention instantly. Im a freshman and heres how i start convos with all the junior and senior chicks.
(props to Fingers and his breath taking as well as educating post)
Me: Hi, excus my interuption but I'm doing this sociel experiment to overcome my fear of girls that I find attractive. Thank you for perticipating. *Smile then walk off*
Usualy she would ask me about it, in lot of case I would start feeding her with my presence slowly like a true player, forgive me the right word would be DJ.
If she is not experience or fast, she would probaly act all confuse like most of us do when we come across a interesting girl, but dont sweat it becuase 1. She are not sure how to aproach this situation becuase apperantly this never happend to her and talk about this with her freinds(more rep.) also shes nervous 2. She find an aproach to this and anitiate a convo with you later on. Then its all up to you