Stephen Birch, 53, sexually assaulted the teenager, who cannot be named for legal reasons, at his home in Partick, Glasgow, in December 2011.
Birch attacked the youngster during a piano class as they played Christmas duets.
When Birch was arrested, officers searched his house and found hundreds of pornographic images of young boys on a DVD and a mobile phone.
Police officers also discovered that Birch had worked at the private Glasgow Steiner School.
In 2000, teachers at the prestigious college asked Birch to undertake a check which would have revealed he had previous convictions for indecency against young boys. He refused – and stayed at Steiner until 2004.
Birch was jailed at the High Court in Edinburgh yesterday.
Judge Lord Bracadale also ordered that he be supervised by the authorities for four years following his release.
Birch pled guilty to sexual assault and possessing child porn at an earlier hearing at the High Court in Glasgow.
At that hearing, prosecution lawyer Shirley McKenna told the court Birch had known his victim for several years. The court heard that after he abused him, Birch said he would see him the following week.
Ms McKenna added: "The complainer left the accused's flat. On his way home, he walked past a phone box and went into it. He thought about phoning 999 but decided against it.
"He described the situation that he found himself in as 'weird' and he felt scared.
"The complainer arrived home at about half-past six and he phoned Childline at 6.44pm. He told the operator at Childline what had happened.
"He was extremely upset on the phone. He didn't want to tell his mum what had happened but gave permission for Childline to phone his mum and tell her."
The court heard that Birch was arrested by police on December 7, 2011.
Officers then found illegal porn images on his computer and a phone. They also dis-covered he had previous convictions for indecency and lewd and libidinous behaviour against young boys.
Yesterday, defence solicitor advocate Tony McGlennan told the court his client recognised the "gravity" of his offending.
Mr McGlennan added: "He does feel sorry. He apologises for all of it."
Passing sentence, Lord Bracadale told Birch he had been in a position of trust and had abused his responsibilities towards his victim.
Lord Bracadale added: "You have pleaded guilty to a serious charge. You are assessed at a high risk of re-offending against young boys."
Kathryn Turner, school manager at Steiner School, said: "Before disclosure came in in 2002 there were no regulatory checks in the UK for those working with children and, when it came in, it wasn't retrospective, it was only for new employees.
"In 2000, however, people were beginning to look at the issue and we asked our staff to go through a check. At that time it was not regulatory and even today there is not a requirement in law that a teacher must have a check. The Care Inspectorate is very happy with our current procedures.
"In 2000, pre-Soham, things were generally very different across the UK as to what they are now."