Hatsukanezumi no Jikan

From Kotonoha:

Maki Takano and his three best friends Ryo, Natsume and Mei are students at a private academy for highly gifted children. Like all the other students of this isolated and heavily guarded school, they have never seen the outside world since they were brought to the school at the young age of three.

When a new female student named Kiriko Hinatsu arrives, Maki has the strange feeling that he has met her before. However, he seems to be the only person who can remember the girl. When he confronts her with his discovery, Kiriko reveals the unbelievable truth about their existence: The school they have been living at all this time is actually an experimental facility and all the students are but guinea pigs who are being used for sophisticated neurological experiments.

Maki now has to decide: Will he continue his false but sheltered life or will he help Kiriko escape even if that means putting himself and his mental health at risk?