Three children and an adult are being treated in hospital after they were seriously injured in a school bus crash north of Toronto Saturday afternoon.

Police say about 20 children were on board a school bus on Saturday when it veered off the road, collided with a guardrail and then rolled onto its roof in a ditch on Highway 400 near Highway 88 in Bradford, Ont.

The 50-year-old female driver of the bus had to be extricated from the car, according to OPP Const. Graham Williamson. The children and driver were rushed to area hospitals but none of their injuries are considered to be life-threatening.

Twenty-one other occupants of the vehicle were also injured but were treated at the scene for minor cuts and bruises. Witnesses at the scene said that at one point there were about a dozen ambulances on hand.

An air ambulance was initially dispatched to the scene but was called off when authorities determined the injuries were not life threatening.

Police say most of the passengers on the bus were children between the ages of 9 and 11 years old, along with their adult chaperones. The group was returning home from a bible camp retreat in the Bracebridge area.

The accident forced the closure of several southbound lanes on Highway 400 at Highway 88, located just north west of Newmarket. The roads reopened at around 3:30, after more than a three-hour shut down.

Williamson said the area was closed to allow investigators the space to reconstruct the accident. The cause of the accident is under investigation but Williamson said the rainy morning weather does not appear to be a factor in the crash.