Last Thursday, Oakleigh Greek Orthodox College staff turned up for work to discover the school had been pelted with hundreds of eggs.

The college's 630 students had the day off as professional cleaners took hours to remove the mess with high pressure hoses.

Principal Steve Charisis stood by his year 12 students, saying he didn't "believe it could be one of them".

"I'd be devastated to think it's any of ours because we've worked really closely with our year 12s this year to ensure there is a mutual understanding of the expectations each of us have.

"We've praised the efforts of our year 12s certainly in the lead up to muck-up day.

"I've even had the neighbours say how wonderful they thought our kids have been."

He said that during the previous day, students took part in "harmless" muck-up day events such as the "honking of horns in cars" and "signing of each others' clothing".

The extent of the damage is not known, but estimated to be in the "thousands of dollars".

Senior Sergeant Gary Ogden, of Oakleigh police, said they were looking into the matter.

If the costs exceeded $500, it would be deemed "criminal damage".

"People can be charged for that and get a criminal record. It's sad that young people just don't understand what they do now can affect them in years to come."