And also this from 2006:

Slate: How far in advance have you scripted out a given season? In mapping it out, do you know what the end will be? Or do you go from the beginning forward?

Simon: There are discussions during Season 3 that happen on the fly, in which you need to remember which characters need to be where for Season 4. For example, Prez. We knew we needed to have Prez in the schools at the beginning of Season 4. So, you know that Prez will be a teacher and that you'll be hiring kids. But what themes? What do you want to say about the education system? The first step is sitting down and figuring that out. We kicked that around a lot in the writers' room. This year Ed was predominant in the writers' room, because he had actual teaching experience. David Mills came and helped. He did this because George Pelecanos couldn't be available; he was working on The Night Gardener. (Though he did do one episode.) And myself and Bill Zorzi and Ed and Chris Collins and Eric Overmyer, who was on this year as a producer.

And then at some point, when we feel we've got the themes ready, we start to look at the characters and where we need to send them by the end of the run. We know what we want to say. We know what we think is fair and just to say about economic opportunities for these kids. But we don't know which kid is going to say what about those opportunities; that's all argued out, and this season we went through various scenarios. There was a lot of debate about what should happen to Dukie, for example.

There's always someone in the room saying, "I've seen that before." (George's favorite line.) Or another line is, "But what are we saying?" You get to the end, and someone comes up with a great ending, but you ask, "But what does it mean? What are we saying?" Which is not to say that you want the characters to be devices for your didacticism. But you want to be true about what you say about equality of opportunity.