Eighteen-year-old Andrew Smith disappeared more than two weeks ago. His bicycle and back-pack were found yesterday hidden on a cliff top alongside the Old Harry Rocks, at Swanage, Dorset.
Extensive police searches for him using divers, tracker dogs and a coastguard helicopter have drawn a blank.
The A-level student was last seen at Poole Grammar School on 11 December at 8.40am.
His father, Ken Smith, speaking from the family home in Parkstone, Poole, said: "Until we get him back here we won't know for certain but we are 99 per cent sure he has gone because of pranks and verbal abuse at school. From what we have learned since Andrew went missing it has been going on for about six months.
"Apparently a group of about five or six so-called mates were involved. They would do things like go to a party together and thought it was then a great idea to order a taxi but leave Andrew to make his own way home from about five miles away.
"He never spoke to us about it. He was also given verbal abuse. He had been skipping school for about three weeks before he went missing."
He said he had confronted teachers but had been told bullying could not have happened because it was not tolerated at the school.
Mr Smith, 55, said the family had not had any Christmas celebrations this year. "We haven't had a Christmas this year, it's as simple as that. We did the usual things like cook the turkey but it might as well have been beans on toast," he said.
Mr Smith and wife, Sally, 45, were due to travel to London to launch a television appeal for their son yesterday, while police were launching another search.