A teacher from Indiana is facing a series of child sex abuse charges ... and, she allegedly had a very specific sexual scenario she asked the children to act out using replicas of the killers' masks from the "Scream" movie franchise.

Brittany Fortinberry, a 31-year-old former teacher at Eminence High School, is facing a slew of charges -- including child molestation, sexual misconduct with a minor, dissemination of matter harmful to minors, and more -- for allegedly sexually abusing multiple children she met at two different schools last year.

A new court document goes into detail about the alleged sexual activity ... including an incident described as a "gangbang."

A 13-year-old -- whose name has been redacted in the document -- told a police officer Fortinberry picked him and two of his friends up, and bought them hundreds of dollars' worth of clothes before taking them back to her house.

The boy says he believes Fortinberry -- shown here in a photo from her husband's Instagram page that's since been deleted -- called the kids' parents to see if they could stay the night ... before giving them a dose of magic mushrooms and asking if they wanted to gangbang her.

They accepted ... and say Brittany had only one condition -- that the three kids wear "Scream" masks during the sex. The alleged gangbang is said to have gone on for around two hours.

The affidavit also includes other sworn statements detailing abuse -- including a 14-year-old who told investigators he believes Fortinberry may have drugged him on one occasion.

Worth noting ... several individuals say Brittany befriended their parents and has a son their age -- so, there were more ties between them than the average teacher-student relationship to begin with.

Fortinberry is currently being held in the Morgan County jail.