Mercury actually started the story in Fall 2009. I came in on page 3. At first, about everything was improvised. The first 'planned' piece on my part was the Halloween Bombings to end Chapter 1. So, how to top that ? From then on, all the Chapter ending were given planned 'dramatic' endings (The Clique War, The Valentine Bloodbath [borrowed in part from 'Carrie'], The Townie Attack, and the upcoming Final Showdown).
Several people contributed to the story early on (which is why Chapter 1 is such a mess - I have edited that and added appx. 6,000 words on my edited copy [not yet posted here] that gives greater continuity to the chapter, likewise Chapter 2 has an edited 2,000 words added). Participated fell off by Chapter 3 to just three of us, basically myself, Mercury, & Mike. Several story threads were introduced in the first 3 chapters which I am attempting to bring to resolution by the end of the story.
Some of the new characters it was not my idea to bring in....Notably Kurt....but I have worked stories around them. The characters of Johnny, Beatrice, and Earnest were written out of the the story at the start (not by me....I did send Lola away, however). I brought in the character of Nick from another story at the start of Chapter 3 to give Tony some 'backup muscle', and because I liked the character. Laurent was brought in by Mercury at the start. (I also brought in 'Bob' as kind of a counteract to Russell, who was sent away early but brought back).
It wasn't my intention to have Jake flee the school so permanently, at first, but after the bombings I knew I couldn't bring him back as a regular student. It worked out though, for he has been able to wreak more havoc from without than within. His time, and ultimate defeat, will be the climax of Chapter 5.
In Chapter 2, it became obvious to me that a timeline had to be established and the main plot lines had to be somewhat planned, and the overriding plot line became the corruption of Bullworth and the Vale Valley. Much of the smaller stuff is on the fly (for example, I didn't really plan on putting Kurt and Pinky together, it just sorta happened, Stockholm Syndrome -like). My main task is to now bring all the plot lines together to a conclusion, so no more new plots are going to be written. And yes, I plan on resolving the plotlines of Kurt, Damon, Algie, and Lance.
So, I guess the answer would be....It's a bit of both.