Being one on one takes more effort than being in i group, I agree. I used to have a hard time with this, and heres what I learned to do in these situations.
Ask the other person about his/her life. Like, "so what are you doing this weekend" or something like that. People generally like to talk about themselves, not to say everyone is selfcentrered, but its what they know so its easy to talk about. It can be a great conversationsstarter, remember they will probably ask you back.
Also keep in mind that if the other person asks you a question, its polite to ask it back. Not with all questions ofc, but if they ask you something like hows school going, you are supposed to ask them back after responding. (Maybe its just me who sometimes forgets that)
Possible conversationtopics, new movies (always works, everyone loves movies:)), gossip (I find it incredibly boring to talk about this, but if im desperate I can bring it up "did you hear whatsherface and thatrandomguy is dating?"), plans for the summer, music, recent events.
Remember its not all up to you to keep the conversation going, so relax. Its a two way street. Dont feel bad if it gets quiet and a bit awkward, it happens to everyone. Including NT's.