I've had a story dancing just out of reach of memory for some time now. I keep thinking Vernor Vinge "A Fire Upon the Deep", but the Tine's world had breathable atmosphere so I don't think it's it.
The part I'm remembering is a generation ship type setting, one of the youth becomes delusional and doesn't believe they're in an actual spaceship, some fraction of the other children start to believe him, I think resulting in an attempt to "leave the simulation", resulting in disaster.
Edit: to elaborate further, the children are classmates. The one who has the delusions gets a following of students, causing the student body basically to take sides forming two factions. There are adults onboard but they either aren't aware or dismiss the children's beliefs as harmless. This was a full length novel, this plot line taking place in the middle third of the novel (approximately), and I read it 3 to 5 years ago, but the novel may be older than this.