Eleven teenagers have gone on trial in the German town of Hildesheim, accused of abusing and humiliating a classmate and filming his ordeal.
The case sent shockwaves through Germany
The students, aged 16 to 18, allegedly beat and kicked their 17-year-old victim, forcing him to strip, eat chalk, and chew cigarette ends.
The case caused an outcry in Germany when grainy video footage of the incidents was shown on TV in February.
If convicted, the students could face up to five years in prison.
They are accused of a wide range of offences including causing bodily harm and coercion over beatings inflicted on the victim from September last year.
The 11 defendants, all dressed in jeans and wearing baseball caps, seemed nervous and often looked at the floor as they waited to go into the closed session at the Hildesheim juvenile court, the German magazine Der Spiegel reported.
Prosecutors say the victim's classmates at the Hildesheim school knew about or were involved in what they call the "systematic torture" that began two weeks after he arrived at the institution.
The alleged acts include forcing the teenager to expose his genitals, and punching or beating him with objects including sticks and a screwdriver.
The alleged abusers videotaped their actions, which are said to have taken place in a classroom and a storeroom at the school; some of the pictures were distributed on the internet.
The victim's ordeal became an open secret in the school, prosecutors say, but it was months before he plucked up the courage to tell a counsellor about the abuse he was suffering.
Prosecutors say the teenagers have all confessed but one of the defence lawyers, Henning Sonnenberg, said none had been able to explain why they acted as they did.
The four alleged ringleaders have been in custody since early February.
They were all students at a vocational school in Hildesheim, near Hanover, on a one-year course to prepare them for jobs.
The victim, who is undergoing therapy, was not present for Tuesday's hearing but has been asked to give evidence next week.