Text bullying at one of Christchurch's top schools ended with one student facing multiple sex charges and his younger classmate on the brink of suicide, a court has been told.

A 16-year-old student at the school, which cannot be identified, is accused of using a stolen cellphone to pose as a wealthy and vindictive drug dealer whose threatening text messages terrified a 15- year-old boy into repeatedly submitting to sexual indecencies and spanking.

The 15-year-old thought the defendant, his best friend, was also a victim of the malicious threats by the mysterious stranger they nicknamed Goth.

For three months, he submitted to increasingly serious and frequent sexual contact with his friend, believing the 20 to 30 text messages a day were from Goth.

The case adds to evidence of an epidemic of text bullying in New Zealand schools that has been blamed for the death of a 12-year-old Waikato girl and prompted children as young as seven to be among 500 callers a day to the What's Up children's helpline.

The 15-year-old gave evidence to the Christchurch Youth Court in statement form. He said his ordeal began in September 2004 when he had a drunken one-night stand with a schoolgirl at a party hosted by the defendant. The girl told the court the 15-year-old never mistreated her and accepted it when, after a week during which the defendant tried to turn her against his classmate, she sent a text message ending the relationship.

The defendant then began repeatedly implying to the 15-year-old that the girl was saying he had sexual problems. The defendant offered to solve his problems and tried to teach him with a hands-on demonstration that left the boy uncomfortable and vowing it would never recur.

"I am not, in any way, shape or form, homosexual," he told the court.

In January last year, he began receiving threatening text messages. He spoke with the defendant, who said he was receiving similar messages ordering them to have sexual activity together. "He told me to ignore it. (Later) he told me he'd asked around and found out someone he called Goth was responsible," the boy said.

Goth's detailed knowledge of the inside of the 15-year-old's three- storey home in a Christchurch hill suburb and his ability to know exactly what the two schoolmates had been doing made him believe the threats, including damaging his home and his father's company cars, were real.

Terrified, he said he submitted to the sexual activities dictated by the messages.

"I hated what I was doing but did it because Goth told me I had to. If I didn't do as I was told, I believed the threats he made would be carried out," he said. "That's why I didn't go to the police or to my family. I totally believed what was being said to me by Goth. (The defendant) gave me the impression he hated what he was doing but felt he had to."

When valuables were stolen from his bag at school in March last year, he believed Goth's claim that he had contacts at the school. The school's deputy principal was alerted and he sent the boy to a counsellor, but the texts continued.

The defendant allegedly suggested that if they continued to submit to Goth's threats, they would eventually be left alone, but instead the text threats became more serious and included specific orders for spanking and more serious sexual offences.

"I was so angry at Goth and the fact I felt powerless to do anything to stop it," the 15-year-old said. "When (the defendant) spanked me, he hit me as hard as he could. I had to put something in my mouth to bite on to fight the pain. I'd wait until the pain subsided and tell him it was OK, then he'd spank me again. And so it went on."

The 15-year-old began to suspect the defendant was somehow involved with Goth, "but even then I didn't ask him because he was my best friend and I didn't believe he would do something like this to me".

Eventually the boy became depressed and suicidal. On April 30, he broke down and, after talking to his father for an hour, admitted what had been happening. The police were called.

The court was told the text messages continued. "It was from Goth's number. I looked at them and I was scared that Goth was still out there and was going to carry out the threats he made," the boy said.

The court was told that the defendant's father, a Christchurch businessman, later went to the police with an empty cellphone box that had allegedly been found in the defendant's room. On the outside was the number supposedly used by Goth.

The defendant's parents were present at the police station when he made what the court was told were "full and frank admissions" and were in the Youth Court this week when their son was committed to stand trial in the High Court.

Court rules ban identifying the school of any defendant in a Youth Court proceeding of any sex-charge victim under 17.

Judge Jane McMeeken committed the defendant, now aged 17, for trial on charges of unlawful sexual connection, two charges of sexual violation and two charges of indecent assault.