The mother of the teen accused of opening fire in a Georgia high school — killing multiple people and injuring several more — has been arrested … but it has nothing to do with the early September massacre.

Marcee Gray … mother of Apalachee High School suspected shooter Colt Grey … is accused of elder abuse after allegedly taping her mother to a chair and leaving her for nearly 24 hours, per the Atlanta Journal-Constitution, citing an indictment.

Deborah Polhamus, who is 74, was allegedly trapped until someone found her in her Georgia home last November. The brutal confinement was allegedly punishment for Deborah refusing to go with her daughter to confront her ex-husband, Colin.

Marcee reportedly faces up to 20 years in the can … after being indicted on single counts of exploitation and intimidation of a disabled adult or elderly person, false imprisonment, criminal damage to property in the second degree, and theft.

As TMZ previously reported … Marcee made national headlines after her 14-year-old son was named the suspect in a high school shooting that killed two students and two teachers and left nine others injured.

Marcee made a frantic phone call just before the shooting, telling school officials she got a call from her son and she feared the worst … when she got a text from her son which read, “I’m sorry, mom.”

Colin, the suspect’s father, also faces several charges stemming from the school shooting.