Droves of teachers at top-performing private schools are signing up to mark other students' VCE exams in a bid to gain valuable insight into the high-stakes tests.

Documents obtained by Fairfax Media under Freedom of Information show that some of the most elite private schools have the most VCE assessors.

Haileybury College has the most VCE assessors in Victoria, with 68 teachers at the school's campuses marking exams in 2015.

The independent school has an impressive median study score of 35, and 24 per cent of its study scores are 40 or above.

It was followed by Caulfield Grammar which had 52 teachers working as assessors, Xavier College with 38, Methodist Ladies College with 35 and Carey Baptist Grammar with 35.

Haileybury principal Derek Scott said around 30 per cent of VCE teachers at his school were assessors, and they were actively encouraged to sign up.

"There's no doubt that you get a better understanding of what the marking process is and what they are looking for and you can adjust your classroom practices," he said.

"We see that there's value in it."

Haileybury's 68 assessors include seven for French, five for physics and further maths and four for Japanese and music performance. However in 2015, Haileybury did not have any assessors for English, the most widely-taught VCE subject.

A Victorian Curriculum and Assessment Authority spokesman said there was a "general association" between high achieving schools and larger numbers of assessors.

"Experience as a VCE examination assessor provides teachers with valuable professional development," he said.

"It gives teachers the opportunity to discuss in detail the range of examination responses and make an important professional contribution to the marking of VCE students."

But there are safeguards – assessors cannot identify the student whose paper they are marking, are never given their own students' papers and each paper is marked by two assessors.

It's a sought-after position and last year 3553 of the 4952 people who applied to be assessors were successful.

Around 83 per cent of schools had at least one teacher marking the VCE in 2015, and about two thirds of VCE assessors in Victorian schools also taught the subject they mark, a Fairfax Media analysis found.

The link between assessors and top grades is apparent at St Kevin's College, which achieved 10 perfect scores of 50 in VCE English last year and 107 study scores above 40 in the subject - making it one of the top schools for English.

Four teachers at the school were also VCE English assessors in 2015, something only two other schools could boast.

The Toorak school's director of studies, Gary Jones, said that VCE assessors passed on their knowledge to other teachers.

"They have that strong working knowledge of what the VCE study designs are asking students to do."

Bendigo Senior Secondary College has 33 assessors, more than any other state school in Victoria. The large school had the highest concentration of assessors in the subject that it achieved the most 40-plus study scores in, Further Mathematics.

Principal Dale Pearce said there was "no doubt" that assessors improved student grades.

"It's is terrific for those teachers to understand how the assessment works, they get to see the standard that is required, and then they can use that experience to enhance their teaching, and then they can go and work with other teachers."