A Christchurch bus company boss is investigating why one of his drivers tried to drive a school bus under a low overbridge, injuring seven children.

About 20 students were on board the bus travelling along Richardson Terrace when it crashed into a rail overbridge smashing the front window on Tuesday morning.

Seven children and an adult were left with minor injuries from broken glass and taken to Christchurch Hospital as a precaution.

The bus is about 3 metres high and the overbridge is 2.4m from the ground.

A police spokesman told NZ Newswire the area is signposted to show the vehicle height limit.

Red Bus chief executive Paul McNoe told NZ Newswire the driver had been taking the group from Woolston School to Ferrymead Park and had not taken the normal route.

He was unsure why the driver, who has worked for the company for about three months, had decided to take a different route but said an investigation is underway.

"I am pleased there were no serious injuries. There could have been so clearly he approached the area with caution and slowed right down - short of not going under the bridge he did the next best thing," he said.

"The driver is a good driver with a good record, no issues. Nothing to tell us there was a potential for this."