David Lowe, 61, allegedly targeted 10 separate victims - who cannot be named for legal reasons - in the historic allegations which relate to a period between 1978 and 1984.
The attacks are said to have taken place against eight to 13-year-old boys while he worked at Westminster Cathedral Choir School and Ampleforth in Yorkshire which is run by Benedictine monks.
In a video interview played to the jury, one of Lowe’s alleged victims told Southwark Crown Court in London that he was around 10 or 11 years old when he first met the former housemaster.
Speaking of his three years at the choir school, he said: “During that time I was sexually abused by David Lowe.
“There was also another incident after I left that school, when he moved from that school up to Ampleforth College in Yorkshire.
“During the course of those three years I suffered repeated sexual assaults by David Lowe.”
The man, who cannot be named for legal reasons, explained that when he first joined Westminster Cathedral Choir School he had been very upset and homesick and Lowe had come across as being very “warm and helpful” towards the children.
The alleged victim said: “My recollection of him was someone in his twenties who was friendly and supportive.
“Over the course of the first year he did a good job of being friendly and supportive and trying to help me. But also there was a more sinister side to it, which I now understand.
“He befriended me, I took him into my trust and vice versa, and certainly by the end of the first year he had established that I was one of his kind of special pupils.”
The man explained that Lowe had a small group of “special boys” that he was extremely friendly with, in whose lockers he would leave little treats, including sweets.
The alleged victim said that in his first year at school Lowe had “established a relationship” with him, and although he did not sexually touch him, he would approach him as he practised the piano and massage his shoulders.
Jurors then heard that in his second year at the choir school Lowe began visiting the youngster in his dormitory at night and touching him intimately under the covers.
The man said: “When he would start off he would sort of whisper ‘I think you’re a very special boy’ and ‘I want you to relax’, and would congratulate me and he would kiss me on the head.”
He added that although he had not been aware of anyone else at the school who had also been molested, he remembered a conversation with a group of boys in which a number of them said they had been left treats by Lowe.
When asked why he had not come forward about the alleged abuse sooner, the man said: “I buried all of the experiences - not completely - after they happened.
“Again, deep shame and guilt and a feeling of humiliation. And I just told myself that it had no effect on me at all.”
He added that having children himself had made him realise that “no child should ever have to go through what I went through”.
The court heard that the alleged victim had seen that Lowe had been working at a sixth form college.
He told jurors: “I think he should be made accountable for what happened, and I think that other children need to be protected.”
Retired Lowe, from Bristol, denies 15 counts of indecent assault of a boy aged under 14.