I usually think of a GM's primary job as being the plot guy. In most cases I'm not worried at all about players getting unruly or anything, I'm just worried about keeping the game going. I also think of them as a sort of keeper of secrets. You need a GM if you want effective plot twists or surprises. 

Even in NYAMA(a three player rp with Eyeris and another long gone player) was mostly collaborative, but I always had something a bit closer to my chest.

A GM can also be a good partner for one of the other players if they want to keep a side plot shadowy for a while. I've added details to the plot at the behest of either player, with the other not knowing. 

I digress.

We're definitely moving to older players, I think. I suppose the question is how old. Who wants to be in school, and who wants to be out? And is anyone vehemently opposed to either?