Recent incidents of sexual violence have sparked protests in several Indian cities this year, echoing mass rallies over a fatal gang rape case in 2012, and putting pressure on Prime Minister Narendra Modi, who has promised action.

In the latest case, five students, a teacher and school principal were arrested in the eastern state of Bihar after a 15-year-old girl filed a complaint last week, police said.

District police chief Har Kishore Rai said the girl told police she was raped by students in a school lavatory in December 2017. The sexual abuse continued for seven months, she told police.

The victim said the principal and two teachers allegedly raped her when she reported the incident to them, Rai said.

"She has alleged that over a period of seven months, 19 persons have committed rape on her," Rai said, adding that 16 suspects were students at the school.

"The investigation is progressing, we are looking into the evidence against each of the accused," Rai said.

Opposition parties in Bihar have blamed the state administration for failing to protect young women and girls.

"Imagine a minor girl being raped by the school principal and teachers along with her classmates. How will you rate law and order of Bihar?," said Shakti Yadav, spokesman for the opposition Rashtriya Janata Dal party.

State official Arun Kumar said schools have been asked "to take necessary measures to ensure safety of girl students".

Activists nationwide accuse authorities of failing to protect women and of moving too slowly to investigate and arrest perpetrators.

A sexual offence is committed against a child in India every 15 minutes and crimes against minors have jumped more than 500 percent in the last decade, an analysis by non-profit Child Rights and You showed last month.

Activists say violent crime against women is rising in India despite tough laws enacted after the 2012 gang rape and murder of a student in New Delhi sparked national outrage.

India's Supreme Court on Monday upheld the death penalty for three men convicted in the 2012 rape case.