Parents have long blamed their children's slipping grades on poor teaching, issues at home or technological distractions.

Now they can point their fingers at controversial changes to school reports.

The Education Department has overhauled the way schools grade students in their reports, making it more difficult for children to achieve exceptional results.

Sought-after A grades – or equivalent top results like "outstanding" – are now only awarded to students who are 18 months ahead of the expected level for their age.

Students who are six months to a year ahead now receive a B rating, and those working at the expected level receive a C.

Schools have been scrambling to explain the changes to confused parents, some of whom mistakenly thought that their children had gone backwards when they recently received their mid-year reports.

Under the previous rules, A grades were awarded to students who were 12 months ahead of the expected level of their peers.

"It is going to be harder for a student to receive a grade that demonstrates that they are doing better than the expected level," Victorian Principals Association president Anne-Maree Kliman​ said.

She said principals had been carefully explaining to parents that their child's performance hadn't necessarily slipped.

"The way it is now scaled means that it is harder for them to demonstrate a higher achievement," she said.

Henry Grossek​, the principal of Berwick Lodge Primary School, said the bar had been set too high.

"The consequence is that less children will be getting an A, and we are having to re-educate parents and children," he said.

"It could be viewed that we had to lower people's expectations on achievement. It has been made more complex."

He said that there were now just a handful of students receiving As in each class at his school – sometimes none – compared to scores of students last year.

Karen Sinclair, a parent at the Berwick state school, said her high-achieving daughter was disappointed when she opened her report.

She had gone from receiving As last year, to receiving Bs.

Concerned about the apparent decline in her daughter's performance, Ms Sinclair organised a meeting with teachers at the school.

"She works very hard, she enjoys school and gets involved in everything," she said.

"Without seeing that A there, she felt like a failure."

The changes were brought in at the start of the year as part of the new Victorian curriculum, but many parents did not notice them until mid-year reports were distributed in June.

Schools were still receiving information about the changes in May.

An Education Department spokesman said the previous reporting algorithm had been changed so that there was more consistency across the scale.

"The department is encouraging all schools to focus on reporting student learning growth as well as how a student performs compared to the expected performance for their age," he said.

He said the department was working with schools to help them communicate with parents about the changes. 

Schools no longer have to report using traditional A to E gradings and can create their own five point scales. This may include descriptions that range from unsatisfactory to outstanding.

The changes affect all reports, because students must be achieving at a higher level to be placed the top of the scale.

Many schools, including Toorak Primary School, have warned parents against comparing this year's report to previous reports.

Parents Victoria executive officer Gail McHardy said the changes were confusing for some parents, and needed to be explained better.