Title of an anime with children quarantined in a city block? This is all I can remember:

It's an anime series with some SF or future theme. The government has the ability to close off a very large city-block with walls that can move and destroy buildings. Behind these walls, they have defenses which can kill anyone who tries to leave the quarantined area. As far as I know, there was some form of virus, but the surviving children were not contaminated. 

As far as I know, it's about a high-schooler who has to survive in this quarantine and has to form a society to survive. At first, the leader wanted to be more democratic, but then his girlfriend gets killed which turns him into become a hard-line dictator. In the end (I think, not sure), they build enough resources to make an attempt to escape.

I'd like to know the title of this series. For some reason, the new Valvrave anime reminds me of this series.

I hope I've given you enough information to find the title.
I think you are looking for Guilty Crown