A mother and her teenager daughter were killed when a driver, who had been seen weaving through traffic, smashed into their car as they left a school in Melbourne's south-east on Wednesday night.

Teenager Xinyu Yuan and her mother, Ma Li Dai, were leaving a parent-teacher conference at Lighthouse Christian College in Cranbourne East when their Toyota was struck by a Holden Commodore.

They were pulling out of the car park, about to turn onto the South Gippsland Highway near Devon Road, when they were hit.

Police say the car had been seen weaving through traffic and the driver may have been drinking.

"We believe the Holden Commodore has left a licensed premises in Cranbourne prior to the crash," Detective Senior Sergeant Gerard Clanchy told Seven News.

The school's principal, Jacob Mathews, said that there had been at least four similar accidents with cars leaving the school, including a serious one three weeks ago involving a teacher.

Channel Seven has reported that Xinyu's father was unable to attend parent teacher interviews because he was working and rushed to the scene when he heard about the accident.

Mr Mathews said Xinyu's father was distraught over the loss of his only daughter. "It's a life gone and the father is devastated. It was his only daughter,"he said.

According to Mr Mathews, Xinyu's 24-year old brother is refusing to believe what has happend. "He is in denial. Saying 'no it couldn't have happened'."

Xinyu Yuan was in Year 8 at the school. Her mother was in her 40s.

Mr Mathews said the school community was "gutted" by the death of the "clever" student.

"She really liked the school and did very well here. She was very happy. There is a radiance about her, you look at her picture [in the school yearbook] and you can see it.

"She is always glowing. We just had a class teacher say 'I'm going to miss that'."

"She was a lovely girl. She was just so gentle and so good. She was a very kind young girl and very clever. We will miss her," he said.

"It's affected us very deeply. Many of the staff are crying. She was one of our daughters. She joined us last year."

Mr Mathews has been trying to get the speed limit on the highway as it passes the school reduced to 80km/h for at least seven years.

He's previously met with council and VicRoads but says VicRoads told him they were reluctant to reduce the speed limit on a major highway, even for just a narrow section.

More than 800 students attend the school, and Mr Mathews estimates there are 150,000 entries and exits to the school every year.

"They didn't want to slow down the traffic. But it's only going to delay you half a minute. I think 80 is reasonable."

Last month a teacher leaving the school was hit by a truck at 80km/h but survived with broken ribs.

"She was pulling out, it was about five in the evening and the sun was in her eye," Mr Mathews said.

The principal said the school was "crawling with people" at the time of the accident and several children and parents witnessed the crash.

"The kids were crying, the parents were crying. This was a terrible scene," he said.

The family's former pastor, the Reverend Scott Ang of the Chinese Methodist Church in Preston, said he'd known the family for seven years, ever since they came from China to work.

"They are very nice people, very good people," Rev Ang said.

"Someone said they drive very safely. I am very sad today. I went to their house yesterday, the father was very sad, crying.

"She was a very good student. My kid was in the same class, year 8. My kid was very sad this morning.

Xinyu, Rev Ang said, was a "hard-working, obedient, very nice sweet girl."

Year 8 student Xinyu Yuan. Photo: Supplied

The family, who live in Lynbrook, were regular churchgoers, he said. The father works in a factory nearby as a welder, the mother was a vegetable packer.

The woman "loved her family, her kids, her work, her husband," Mr Ang said

"She was a very good, very nice woman. I've been friends with her for many years."

Mr Ang said there was no signage warning drivers that there was a school ahead.

"The first time I came to this school I didn't even know it was a school," he said.

"There was no sign. Maybe if there was a sign everybody would know to slow down. We need to have signs to warn them to slow down."

Weaving through traffic

Police say the driver of the Holden, a man in his 20s, was seen overtaking a number of vehicles on the highway before the crash occurred about 7.20pm on Wednesday.

Detective Senior Sergeant Gerard Clanchy said witnesses had told investigators the Holden was seen weaving through traffic.

"We had some reports that the silver Holden Commodore was seen overtaking vehicles on the South Gippsland Highway prior to the crash and we'd appeal for any other drivers or witnesses who made these observations to contact Crime Stoppers right away," he told Channel Nine.

The man had to be cut from the wreckage and was taken to The Alfred hospital in a serious condition.

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

The Cranbourne East crash is the latest in a string of deadly collisions, with six people dying on Victoria's roads in as many days.

"Too often, police are seeing carelessness or recklessness leading to tragedy on the road," Assistant Commissioner Tim Hansen said.

Anyone with any information is urged to contact Crime Stoppers on 1800 333 000 or www.crimestoppersvic.com.au