Witnesses told CBC News the boy is a student at the high school on Terrace Street.
The incident happened just after 2 p.m.
Emergency crews are on the scene and police are advising drivers that there will be delays.
In a release, the Cape Breton-Victoria Regional School Board confirmed the incident involved a student from Sydney Academy and a bus that serves the high school.
Spokeswoman Michelle MacLeod said after the incident that other students were rerouted through the school and dismissed.
MacLeod said Thursday classes have been cancelled, but the school will be open and crisis teams will be available.