He pleaded guilty at his arraignment in West Roxbury District Court today. Prosecutors had sought bail of $200,000, according to the Suffolk County District Attorney's office. His bail was revoked on an unrelated juvenile matter.

In a statement, DA Dan Conley said: "The defendant had a cocked and loaded 9mm handgun inside a school. With rounds in the chamber and the magazine, it presented an extremely dangerous situation in a building full of innocents." More from the DA's office:

Police, prosecutors believe that Oliver let West, Rutledge, and the juveniles into the school through a side door yesterday morning. Together, they allegedly went to an upper floor of the building and there became involved in an altercation with students.

Boston School Police responded to the altercation and apprehended West after a struggle. The other defendants fled the scene but were quickly apprehended by Boston Police, School Police, and an MBTA Transit Police officer assigned to the School Police Unit.

Seized during the incident was a distinctive black, grey, and red backpack West had been wearing. Inside was a Sturm, Ruger & Co. P95DC handgun with its hammer in the cocked position and rounds in its magazine and its chamber.