During her Panorama interview last week, the Princess said "a young boy" was the source of up to 300 nuisance calls made to Mr Hoare's home in 1992 and 1993, but she did not identify the boy.
Inquiries by the Independent on Sunday have established that the calls were made by a 16-year-old pupil of Stowe School in Buckinghamshire. It is understood the boy, who has since been expelled, made the calls to one of Mr Hoare's sons, either Tristan, 17, or Damien, 15, who attended the school.
The Princess's insistence that she did not make the calls was an important part of her defence against suggestions from what she called "the enemy" - the Prince of Wales's friends - that she was unstable.
The story of the calls broke last year after the Princess was implicated in a police investigation into nuisance calls made to the London home of Mr Hoare over an 18-month period. It emerged that the Princess's private line at Kensington Palace, her mobile phone, the home of her sister, Lady Sarah McCorquodale, and several call-boxes in the area featured on the list of numbers being looked at by police. She denied making the calls, in which the caller said nothing before replacing the receiver.
Mr Hoare, 50, called in the police in October 1993, and police say the calls stopped last March, but not before the Princess's reputation had been damaged.
In her Panorama interview, the Princess admitted making some calls to Mr Hoare, a family friend, but she added: "I was reputed to have made 300 telephone calls in a very short space of time, which, bearing my lifestyle at that time, made me a very busy lady. No I didn't, I didn't. That again was a huge move to discredit me, and very nearly did me in, the injustice of it, because I did my own homework on that subject and consequently found out that a young boy had done most of them."
A source close to the Princess said: "There appears to have been some conflict between one of Mr Hoare's sons and another boy at the school, who is now 17. The latter made a lot of calls from phone boxes in the Kensington, Bayswater and Notting Hill areas of London. These were the ones that some people suspected Diana was using. There was some bullying and it appears the calls were ... a form of harassment."
Stowe School refused to comment. A spokeswoman said: "We have students removed from time to time, just like other schools. But our policy is not to disclose who they are or why they were removed."
Further reports, pages 6 and 7