Andrew was afraid of disappointing his parents. So he left a 5-page letter and ran away.

Eleven-year-old Andrew Ong got in trouble at his primary school.

So, around 8:45am on Monday, he wrote a five-page letter to his parents explaining he was leaving, gathered his backpack and myki public transport card, left the school lunch that had been packed for him and ran away from his Camberwell home.

Nearly 12 hours later, Andrew was found at Melbourne Central train after a frantic search by his parents and police, The Age reports.

Earlier in the day, his mother, Cindy Ong, had pleaded with the public to help her find her son and bring him home.

Cindy said her 11-year-old boy was "upset with himself" after getting in trouble at at Hartwell Primary School, where he was a student.

"That's why he's taken off. We just want him to know it's OK, he's not in any trouble, just to come home," she said.

"He left a note lying on the table about why he's doing it, about how he feels sad about what has happened at school. He just wanted to have some time to think. And he'll come back Friday."

Cindy said her son was probably feeling "a bit daunted" about going to school after getting in trouble.

She said her son was on canteen duty with his friends when they got in trouble for "giving treats to their friends".

"The principal made them write a letter. So I think he feels really bad about what he's done," she said.

"He probably feels that we're disappointed in him."

Partway through the press conference, police presented Andrew's parents with a photo of their son at train station near their home.

"Oh my god. He brought his jacket. That's him," Cindy said.

A car was sent to collect Andrew from the station so he could be reunited with his family, where they assured him he was not in trouble and could never disappoint them.