The bus driver reported that hearing what sounded like a firecracker or small pop when the gun was fired.
Officials said that the boy was struck in the leg.
Petersburg High Principal Alicia Fields sent a letter home to parents three days after the incident on Thursday.
"Today, it's been determined that around 12:30 p.m. on Monday, March 16, 2015, a high school student who was riding bus number 61 had a concealed weapon. During the transport, the gun discharged -- striking the student in question," Fields wrote.
The letter stated that police and high school administrators are conducting a joint investigation.
Fields told parents that in the wake of the shooting, school leaders are "vigorously reviewing the incident, as well as policy and procedures."
School leaders could not say where the student got the gun or why he was carrying the weapon gun and brought it onto the bus.
“Obviously, we are taking this incredibly seriously," said Nicole Bell, a spokesperson for Petersburg schools. "There are safety measures in place the high school. There are metal detectors. There are wands. We have school resource officers here in our building.”
There has been no word on the extent of the student's injuries, but CBS 6 News is told the student has not been back to school since the incident.
Officials said the delay in notifying parents was because the information was not confirmed until Thursday.