So I'm a pretty good conversationist with my friends (male and female), can keep a chat going and I joke around - all the good stuff.

I've been here a while, reading up on expectations, confidence, how to act and what to say, and I can catch a girl's attention.

Also, this may come in helpful: I'm a 15 year-old guy; quite good looking (I think); top-level student, so I have the brains to keep a conversation on many topics and I am relatively cultured. What I'm trying to say is that I can normally catch a girl's attention without saying much.

The story: About a week ago, I went to a party with some friends for the hell of it. Exams have been numbing my brain to the point of near-retardation, so I needed a rest. Walked into the room and immediately done a quick scan of the floor and spotted a few HB8's and a HB9 (teenager standards). I sat down with my buddies on the far side and got drinks, and found out the HB9 is a national model. Attention grabbed. I find her standing with a gaggle of girls, standing rather absent-mindedly, just rocking back and forward on her heels. She gazed around and we caught each others look. She done that thing that women do when she's interested (an IOI, IIRC) and turned around. I finish my drink and go to the bar to get another, hoping to the Lord above she notices and meets me there. Fortunately, she does. I open up on a simple line, and casual chit-chat begins. I notice her giving off IOI's rather frequently, and once or twice an IOD, but I had figured out her game so just kept it cool and continued. By now, me a year ago would've have drenched my shirt in sweat and developed a stutter. So I return the IOI's and IOD's and we keep talking. All is going well until we get into the deeper, more open-ended questions. I held my own for a time, but I'm pretty poor talking to strangers as I don't know how they'll respond to a particular joke or statement, so I crack less jokes and begin to stiffen up. I was returning back to my old ways. I said I had to get back to mt friends and made a joke about how much they depended on me. As I was turning to leave, she stopped me and said that I should go to her beach party after the exams were done (another seddit trick; is she on here?) and I accepted and got the directions from her. I left and made some quick rounds to say hi to some acquaintances, then got back to my table, cursing myself.

tl;dr- met a cute girl and was all going well until I couldn't keep the conversation going.

Any ideas on how to rid myself of this, or any other related tips on this matter?