Erin McAuliffe -- a 25-year-old math teacher -- has been arrested for carrying on sexual relationships with 3 male high school students.

The charter school teacher at Rocky Mount Preparatory in North Carolina was taken into custody Thursday and faces 3 counts of sexual activity with a student and one count of indecent liberties with a minor. Two of the boys she allegedly had sex with are 17 ... the other is 16.

Admin staff at McAuliffe's school reportedly tipped cops off and a month-long investigation was launched. All of her sexual activity with the students is believed to have taken place away from the school.