Victoria Police say it’s a miracle that everyone aboard a school bus survived when a truck ploughed into it from behind, forcing it down an embankment on a highway west of Melbourne.

Two teenage girls and two adults were seriously injured in the crash on the Western Highway at Bacchus Marsh early on Wednesday morning.

VicRoad staff were clearing a crash from earlier in the night when the truck collided with the bus, police said.

The school bus was carrying four adults and 27 students in years nine to 11 from Ballarat’s Loreto College to the airport for a trip.

“Quite miraculously, they’ve self-evacuated and other people, I believe truck drivers and that, stopped to assist them,” Detective Inspector Roger Schranz said on Wednesday.

“I would have assumed someone would have passed away out of this entire tragedy. So they’re all very fortunate people.”

Paramedics said they assessed more than 30 people at the scene, with “traumatic injuries”.

One of the teenager was flown to the Royal Children’s Hospital with lower body injuries. She is in a serious but stable condition.

Another girl was also taken by air ambulance to the Royal Melbourne Hospital. She was in a stable condition with upper body injuries.

The scene of a collision between a school bus and a truck in Melbourne. Photo: AAP

A woman in her 40s and a man in his 50s were also flown to the Royal Melbourne in stable conditions. Both are believed to be teachers at the school.

In all, the teenagers and adults involved were taken to six hospitals across Victoria. They included 13 transported to Ballarat Hospital, which declared a code brown allowing it to call in off-duty staff to treat the injured.

Police will investigate whether drugs, alcohol or speed were factors in the crash.

Bacchus Marsh Towing owner Trevor Oliver was on the scene, working on the clean-up from the first accident, when he heard several “tremendous bangs”.

“I looked and saw a coach was involved,” he told the ABC.

“Your heart absolutely sinks … but you just kick in.”

Mr Oliver said everyone on the scene, including the bus driver and students, worked to help get the injured out of the stricken bus.

“The girls were brilliant … the staff were tremendous,” he said.

“The girls that got themselves out reasonably easily were triaging those who were dazed and knocked around.”

Melbourne-bound highway lanes were expected to remain closed for the rest of the day as investigators analysed the scene.

Detective Inspector Schranz said the investigation was in its “very, very early days”.

The truck involved in the crash had sustained significant damage.

“It’s destroyed the front cabin – you can barely get into it,” he said.

The driver was yet to speak to police on Wednesday afternoon.

The students were on their way to Melbourne Airport to catch a flight to the US for a dream visit to a NASA space camp at the Kennedy Space Centre in Florida. The trip had been delayed for a couple of years due to the pandemic.

Early on Wednesday, nearby resident Ange Greenland woke to the sound of the air ambulance arriving.

She said the crash site was “pretty nasty” and it was distressing that the teenagers had to walk up the embankment after the crash.

“They walked up the off-ramp to a waiting bus half a kilometre away,” she said.

“I really was very angry because I thought if that was mine, if it was my child … I could not believe how they were treated”.

In a statement the school thanked emergency services for coordinating care and providing triage at the scene.

“Our caring Loreto community has deep concern for the injured and their families and we ask for their privacy to be respected,” it said.

-with AAP