Matthew Beevor, 40, was sentenced at Canterbury Crown Court for the sickening ordeal, which last over an hour. It was described as “something like a horror movie”.
Chillingly Beevor, of Hersden, Kent, had tried to attack the child two weeks before but failed.
Prosecutor Christopher May said: “Beevor was acting strangely, leaning over towards him. When the bus approached Brimstone Avenue, both went to get off. It was then that Beevor grabbed hold of the victim's collar and the youngster tried to push him off.
“The driver then told Beevor to get off and the youngster went home and reported the incident to his mother.”the school grounds to recover a hammer and his jacket.
“Beevor took hold of him and pushed him to the floor,” Prosecutor May continued.
“He took the hammer from the child and put it next to his head. He told him to shut up or he would cave his skull in. The victim was very scared.”
Beevor then subjected his victim to an hour long attack. The boy eventually managed to escape and raised the alarm, however when the police arrived, Beevor climbed onto the school's roof to evade arrest and threw bricks at the officers. After ten hours, Beevor finally came down and was arrested.
Sentencing him, after Beevor admitted charges of sexual assaults, assaults and affray, Judge Heather Norton said: “I can only imagine what this child must have felt locking himself in a cupboard in an abandoned school believing you were going to kill him.”
Detective Constable Andrew West added: “A serious sexual predator, who pursued children, is now behind bars for a very long time.
“Beevor's criminal history began when he was 15 years of age with burglary and theft. He subsequently graduated to sexual offending against young teenage boys.
“He has never had any regard for his victims, their age and therefore vulnerability, and has never given any thought as to the devastating lifetime consequences of his crimes. He is a very dangerous man. When he is eventually released from jail he will be subject to many restrictions that will greatly impact on his quality of life so I am very satisfied with the outcome.”