Francis Artey, 27, was driving the bus at 5 p.m. Monday when it left the road, cut across a yard and smashed into a house, police said.
Police Chief Andrew Sluckis Jr. told the Worcester Telegram the bus was traveling 38 mph in a 30 mph zone moments before the crash.
The bus' brakes were not applied, and Sluckis said Artey may have suffered a medical problem before the crash, or the brakes may have had a mechanical problem.
The residents inside the home all escaped serious injury.
Four passengers who were on the bus were not seriously injured, while Artey remained inside of the vehicle for more than an hour before he was extricated and taken to the hospital in critical condition and police have not been able to interview him.
The investigation of the crash is continuing.
John Carney, the general manager of RTA Transit services, told the Telegram Artery went through an eight-hour training program before driving Monday.
He also is a former school bus driver, Carney said.