The girls were held last week and later released on bail, Tandahimba District Commissioner Sebastian Waryuba said.
He said police were looking to question two other girls in the district, which is on the Mozambique border, so they can help with information on the men who impregnated them.
Tanzania has one of the toughest laws against those who impregnate school-aged girls, sentencing them to at least 30 years prison.
In Tanzania, if you become pregnant as a schoolgirl, it means the end of your education. Even though successive governments have made a push for girls education, those who become pregnant are routinely expelled from school and prevented from returning.
Officials said the penalties are intended to discourage early marriages and teenage pregnancies that lead to school dropouts. However, the authorities have been highly criticised for not allowing girls back to school after they have given birth.
In June last year, Tanzania’s president John Magufuli faced condemnation after sending out a threatening warning. He told a rally of schoolgirls: “After getting pregnant, you are done.”
In 2002, a law was passed which allowed girls to be expelled from school for getting pregnant. The law says girls can be expelled and excluded from school for “offences against morality” and “wedlock”.