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Do you know anything about them other than attractiveness? For example if you know they like a certain kind of music and there's a band playing soon that plays that style try inviting them out, depending on style you'll get extra points if it's a local band(and they're good) some people are impressed when you show them underground stuff that they wouldn't normally be able to find on their own. First thing I would do would be to approach them, you said they already kind of know you, so it's not really a reintroduction, catch them like right after class, walk with them a bit make small talk, she'll be with her best friend so try not to focus on talking to just one or the other, talk to them both and make them both feel equally as important(for now), then look at your watch pull the "hey I have to take off" have to get to other class/have to meet up with friends or whatever excuse you have to cut the conversation short. Then say "hey me and some friends are going out to" pick some place, club, bar, music venue, or wherever. Then invite them out, the key is to invite them both out, you're not trying to pick up her friend but you don't want to single her friend out either because if she thinks about hooking up with you it's going to be that friend who's giving her advice as to whether it's a good idea or not. Also finish it off with something like "I guarantee you guys will have a blast" if they say yes, or maybe or whatever then finish it up with "what's the best way to keep in touch?" and then you go your separate ways, if you get a number DO NOT CALL OR TEXT RIGHT AWAY wait a couple days text them a day or so before the event to double check that they're going. And once you're there make sure to talk to the girl you're interested in. And last but not least... Good luck

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well in this case you'll have a bit less time to work with since pretty much you're only option is to either make the approach right before class starts, or right before she leaves the classroom at the end of class. what I would do is make an approach, "Hey what's up?" if she looks at you with that 'who the fuck is this?' look on her face, just "let me guess? you don't remember me?" then reintroduce yourself, make some small talk, invite her out some place to 'catch up' or something like that, don't make it a date type place, no dinner, and forget movies, something like going out for coffee, meeting up at the local mall, some place casual and public.