This was only about 5-6 years ago but there was a book I remember reading a lot of in High School.
It was about a girl and her little brother who lived in this sewer city beneath another city, there was a lot of cement.
Their school bought into this government program to feed the kids a weird paste and then they all were playing VR pod "games" with aerial battles.
Turns out the government was trying to build an army, the siblings find out the people living above them are the uber rich, both groups think outside this cylinder is nothing but death and pollution, but after going over the wall in a plane they find lush forests where the uber-uber rich are living.

  • Hello and welcome to SFF! You have a nice start here but if you can take a look at this guide and edit in any more details that will help us identify you story! – TheLethalCarrot Apr 27 '18 at 8:58
  • Alright, I'm gonna come back when I wake up because I might be able to remember more then. – Sebastyn Devlin Apr 27 '18 at 9:13
  • Also don't forget to register and take the tour! – TheLethalCarrot Apr 27 '18 at 9:13
  • We have this one somewhere on the board. I just need to remember where I answered it. – FuzzyBoots Apr 27 '18 at 9:45

This may be Emma Clayton's The Roar as per Sci-fi book about a male entering a competition that he wins and gets recruited by millitary and Story about boy living in a city sheltered from post-nuclear fallout?.

The Roar - Emma Clayton

From this review:

In the future, a plague spreads through all the world's animals. To survive, the humans create a northern wall through Canada, England, etc. Every living thing south of the wall is destroyed while all the people in the world move north of the wall into multi-leveled cities with no plants or animals. Their food is produced from chemicals and mold.

Because of the lack of space, it's 30 years before anyone is allowed to have children. 2 of these children are fraternal twins Ellie and Mika. When Ellie disappears, Mika's parents believe that she's dead. But the strong telepathic bond Mika shares with his twin leads him to believe that she's alive, and he'll do anything to find her again. This leads him to follow along with what the government demands of him even when he senses there's some secret the government is trying to hide from everyone. This includes joining his classmates in learning war games on simulators and competing in competitions that can make his family very rich.

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