A group of Victorian parents is making a discrimination complaint over religious education in government schools.

One-third of Victorian state primary schools teach Christian religious education, and parents have to opt out if they do not want their children to attend.

But these students are generally not given alternative lessons and are simply supervised by teachers.

Humanist Society spokesman Dan Kerr says four parents have joined the complaint, but many more have raised concerns.

"We started to collate all these phone calls and emails," he said.

"We stopped at around 60 and started to pass those details on to the legal team, because they would then take it over and contact the parents themselves and make that sort of determination."

Mr Kerr says children who opt out of the classes are being discriminated against.

"They actually may not teach the excused kids anything but only supervise them," he said.

"So it's that point where these discrimination cases are starting to appear, where we're hearing stories of kids being left in corridors, they're sitting in the principals office."