It is the first level and one of the key stages of the game.
This is a high school in Detroit. It is a crime scene in the last two years, and police were unable to solve the case even for the absolute lack of evidence, and that all victims showed no signs of violence on their bodies. Because after the slaughter, Professor Brook disappeared without trace and Detroit police concluded that he was the murderer.
The school had to be demolished, but the staff refused demolition because they felt noises and voices inside. This is because there is a secret laboratory in the basement of Professor Richmond where ghosts want to leave the Matrix and ask for help. From this level you can go to all the others through the portal shaped door, but it will on the third level, because you simply have to leave school and go to the New School.
The whole history of this school is explained only when we got to the third level and, more particularly, in the end: the culprit of the murders was the ghost Hawksmoors capture Richmond. The building and even the outdoor garden is part of the level.
The first level is not really over until beginning the third level and have the launch grenade to destroy the obstacles that give access to the New School.