The Girl Who Owned a City/WMG

The adults attempted to set up a system for the children to govern themselves, but it got bogged down and was never completed.
Too many parents trying to make sure their own children got an extra edge lead to infighting and roadblocks.
 * Maybe the "King of Chicago" is part of the system.

The plague is still in full-force - teenagers will die. Mankind is doomed.
It's depressing, but a possibility.
 * We can but hope.

The sequel wasn't published because it included sex
Because most of the main characters are headed headlong into adolescence, with no grownups to keep an eye on them.

There are adults somewhere in a hermetically sealed building at the CDC, still trying to find a cure
I'm sure someone thought of something.