The former Indiana teacher charged with sexually abusing multiple underage children while they wore "Scream" masks has a big adjustment to make behind bars ... no more sex!!!

Brittany Fortinberry is being held at the Morgan County Jail in Martinsville, IN and there's no way she's doing the nasty ... because there are no conjugal visits, no access to porn, and no sexually explicit material -- and she's all alone in her cell.

A rep for the jail tells us Fortinberry is in general population and she has a cell all to herself, despite each room allowing for 3 inmates. It's just her and a metal bunk, sink fixture, towel hooks, toilet, mirror and a night light.

We're told Fortinberry is not allowed conjugal visits and she has no way to access porn on her jail-provided tablet. She has access to some books and magazines, but nothing sexual like Playboy.

Fortinberry's day-to-day is pretty boring ... in the mornings, the doors to the cells are opened and she can mill about with other inmates or hang in her room. There's a TV with cable in the dayroom, where live basketball games are the biggest hit.

She gets three meals a day, but she eats alone in her cell ... with jail staff coming by to slip her a food tray through a slot.

The jail menu typically includes cereal, fruit, cake and a pastry for breakfast; spaghetti, pudding, vegetables, rolls, cake and hot dogs for lunch; and chicken patties, bologna sandwiches, rolls, cake and fruit for dinner.

Sounds like Fortinberry's been on her best behavior in jail ... we're told she has not been disciplined and correctional officers have not filed any reports of bad conduct.

Remember ... Fortinberry is facing a slew of felony charges -- 29 in all -- for child molestation, contributing to the delinquency of a minor, dissemination of matter harmful to minors, and more. If convicted, she faces decades behind bars.