The alleged shooter arrived in an olive green jumpsuit at the Barrow County Courthouse in the rural Georgia town on Friday morning alongside his lawyer ... 2 days after authorities say he opened fire at the school, killing students Mason Schermerhorn and Christian Angulo, and teachers Richard Aspinwall and Christina Irimie. Nine others were injured.

The judge read off the 4 felony murder charges against Gray, and then asked if he could read and write in English -- which he answered in the affirmative.

The judge said given the gravity of the charges and the nature of Gray's alleged conduct, he will be tried as an adult.

Gray faces the maximum penalty for each count ... a life sentence without the possibility of parole.

Gray's attorney did not request bond, but his lawyer suggested he might at a later date.

Gray was previously on law enforcement's radar after he allegedly made a menacing threat about a school shooting on the social media site Discord. He was later banned from the site for violating the platform's anti-extremism policy.

Local officials have yet to reveal whether a motive has been identified. Barrow County Sheriff Jud Smith called the school shooting "an evil thing." Gray is believed to have acted alone ... though, officials are still investigating the case.

Gray's father, Colin Gray, was arrested Thursday evening ... and charged with 4 counts of involuntary manslaughter, 2 counts of second-degree murder and 8 counts of cruelty to children.

The Georgia Bureau of Investigation revealed Colin Gray bought his son the AR-15 used in the shooting for Christmas ... and this was after the FBI visit to their home. BTW, it's illegal in Georgia for a minor to possess a firearm.