A boy who raped a teacher at a school should serve at least 15 years of his life term, a campaign group has said.
The teacher, whose identity is being protected, supported the decision to name him
The 16-year-old, who was 15 at the time, attacked the 28-year-old woman at Westminster City School for Boys in central London last September.
An Old Bailey judge sentencing him on Monday said he would stay in detention until doctors said he was not a threat.
The Victims of Crime Trust called it "incredulous" that the boy could apply for parole after four-and-a-half years.
Norman Brennan, of the trust, called for life sentences to be structured so that "jail terms fit the crime committed".
He said: "Life should mean life. I find it incredulous that an extremely dangerous sexual predator could be out on the streets again in four years.
"The judge should have recommended he serve at least 15 years in prison for the safety and protection of the community.
"This offence was the worst type of rape which in my mind is second only to murder."
The boy, who admitted raping the teacher at an earlier hearing, targeted her on only her second day at the school.
He was caught on CCTV wandering around the premises before the attack.
The court heard that the youth ripped the woman's clothes, bit and hit her during the ordeal.
The teacher put up a fight but the more she struggled the more he attacked her.
After the sentencing the teacher said she was glad to be alive because she thought she was going to be murdered.
She called for her attacker to be given treatment.
In mitigation Jeremy Dein, QC, defending, said the teenager had come from an "emotionally fraught" family background, had been bullied at school and suffered from a misconduct disorder.
Judge Christopher Moss lifted the court order protecting the youth's identity, but this disclosure was due to be delayed until Friday afternoon after the defence said it wanted the decision reviewed by the High Court.