Local reports said some 34 schoolgirls were treated in hospital after a crowd of teenage boys, their parents and locals beat them up on Saturday.
The girls from the government school in Triveniganj had earlier admonished local boys for sexually harassing them.
As of Monday, six boys and three women were in custody, police told media.
They suspect another eight to 10 unidentified people were also involved in the mob attack against the girls, believed to be aged between 10 and 16.
Police said on Monday that several schoolgirls were still undergoing treatment in hospital, but would be discharged soon.
What happened on Saturday?
Students playing on the grounds of their boarding school in Saupaul district, 250km (160 miles) from the state capital Patna, caught a group of boys sneaking in and asked them to leave.
When the boys refused and began sexually harassing them instead, the girls argued back and some physically remonstrated with the boys, who retreated.
Police say a group of the boys then returned with their parents and other locals, armed with bamboo sticks and iron rods.
They broke into the school and attacked students and teachers, who tried to call for calm, police said.
“They dragged us by our ponytails, assaulted [us] with bamboo sticks and kicked and punched,” one of the students treated in hospital told the UK’s Guardian newspaper.
“We were unarmed and had nothing to protect us. I saw many of my friends lying on the ground and crying with pain.”
A staff member at the school told the Times of India she saw “around 15 people, both men and women, thrashing the girls with canes and rods” and damaging school property.
‘Action will be taken’
Local residents took to the streets in Triveniganj to protest against the attack on the schoolgirls on Sunday and national campaigners are now calling for girls in state-run institutions to be protected.
Police say they are investigating claims that the schoolgirls had earlier reported repeated sexual harassment by local teens, who had lurked near their school and written vulgar comments on school walls.
“Action will be taken against all those found involved in the attack on the girls,” Baidyanath Yadav, District Magistrate of Saupaul told local media.
Sexual violence has become a major political issue in Bihar, one of India’s poorest states, particularly after a report earlier this year found dozens of girls had been allegedly raped in a children’s care home.