Jeri Packham is among them, and said her state school asked her to leave in Year 11 after she failed a unit of VCE English. She was battling depression and anxiety at the time.

"They didn't offer me the opportunity to repeat. It was just, 'you failed, goodbye'."

For the first time, the Victorian Ombudsman has shone a light on informal expulsions, revealing that as many as 6800 students are pushed out of Victorian state schools every year.

"We simply do not know where they end up," the report said.

A total of 278 students – mostly boys in Years 8 to 10 – were formally expelled from Victorian state schools last year.

Principals must now seek Education Department approval before they expel students who are eight or under after the watchdog raised concerns about 43 primary school students being expelled in 2016.

Some of these students were just five or six-years-old.

"It is difficult to conceive of circumstances where the behaviour of children as young as five or six could be of such magnitude that expulsion is the only option available," Ombudsman Deborah Glass found.

Almost one third of expelled students had a disability or a mental illness, and Aboriginal children and those in state care were over-represented.

But the actual number of students ordered to leave is much higher.

"Somewhere between hundreds and thousands of children each year disengage from formal education at least in part as a result of pressure from schools," the report said.

Ms Glass said principals were failing to ensure that expelled students found another school.

In some instances, students were disengaged from school for up to 12 months following their expulsion.

"We have parents that have had to give up work because of the experience of not being able to find their child a new school," she told Fairfax Media.

"Principals are simply washing their hands of this."

The report also highlighted the inconsistent approaches to expulsion between schools. Students were ordered to leave for the following reasons:

Repeatedly bringing a phone to class, not completing work and using aggressive language

Not wearing the correct uniform, showing up to school late and using offensive language in class

"Trashing the classroom". The seven-year-old had autism and ADHD.

Hitting an assistant principal. The primary school was aware of significant upheaval in the child's life including Department of Health and Human Services' involvement after her mother tried to relinquish care to her grandmother.

Admitting to smoking marijuana on school grounds. The secondary school student had Post Traumatic Stress Disorder.

The Ombudsman decided to investigate following a 25 per cent increase in expulsions between 2014 and 2015.

The state government responded by announcing $5.9 million program to engage students with challenging behaviour and complex needs.

"We're acting decisively to implement the Ombudsman's recommendations, and support principals, teachers and students."

Schools have created "placement committees" to share expelled students, according to a submission by the Victorian Association of State Secondary Principals.

They "take their turn" to accept expelled students, including those from nearby private schools.

Australian Principals Federation president Julie Podbury said principals went out of their way to avoid expelling students.

"Principals bust themselves to try and find a solution. We find expulsion unpalatable," she said.

Jeri, who is now 17, has been out of a school for a year.

"It's discouraging being in a room of adults who say 'you've failed and there's nothing we can do to help you'," she said.

The principal of Jeri's former school denies the claims and said teachers tried to help her complete the VCE.

But Jeri is optimistic about her future and has enrolled in a nursing course at TAFE, which will start next year.

"It's going to be a bit of a change going back to study, but it should be good."