WINDER, Ga. − Parents fretted Tuesday as nervous students returned to classes less than a week after two students and two teachers were gunned down inside a high school here.

Classes for most of the 15,000 students in the Barrow County School System resumed without obvious incident, under heavy law enforcement presence.

But classes remain paused at Apalachee High School, where investigators say Colt Gray, 14, opened fire Sept. 4 with a rifle given to him by his father, Colin Gray. Colt and Colin Gray have been arrested and face murder charges, and they remain in custody.

Killed were students Mason Schermerhorn and Christian Angulo, both 14, and math teachers Richard Aspinwall, 39, and Cristina Irimie, 53.

"We know the days ahead are going to be difficult, and we have some staff and some students who are not ready to return to school," Barrow schools Superintendent Dallas LeDuff said in a video message to the community.

Authorities say at least six school threats and would-be copycats have been reported across the state since the Apalachee shooting, including a 13-year-old arrested for making online threats and a middle-schooler arrested for bringing a handgun to campus.

Investigators have not yet released specific details of how they say Colt Gray attacked the school, or how exactly he was stopped by two police officers who were there that day. Apalachee High School does not have metal detectors but had recently introduced a new panic-button system for teachers, which investigators say was used by several when the shooting broke out.

Across the district, classes went on largely unremarkably Tuesday, aside from an unusually heavy presence of sheriff's deputies and Georgia State Patrol officers.

At Apalachee High School, administrators and staff were inside the building but the student parking lot remained empty as helium balloons sparkled in the morning sun below the American and Georgia flags flying at half-staff over the campus.

"Ms. Imrie I will miss you with all my heart," reads one note left by a student. "I was starting to like algebra a lot."

Though returning to classes will help restore a sense of normality, the greater community will be affected for decades to come, said Ronn Nozoe, CEO of the National Association of Secondary School Principals.

In some communities that have suffered a school shooting, neighbors have turned against one another as they criticize or defend authorities and parents for the response and recovery.

"These things can tear entire communities apart," Nozoe said. "These things don't go away. Once they happen, you can't undo them."

The Barrow County public school system consists of 10 elementary schools, four middle schools and three high schools. It employs about 2,000 people full and part time, including about 900 teachers and certified personnel.

In his message to the community, LeDuff, the superintendent, said mental health support teams would be available in schools. And he noted there are "a lot of decisions to make" as the district resumes full operations. After many school shootings, administrators either remodel or raze buildings or potentially change alarm sounds or lunch menus to avoid reminding students of specific circumstances surrounding the shooting.

"I want to thank our law enforcement community and our first responders for standing in the gap on a day that … on a day that we never thought we would have to go through as a community," LeDuff said in his message, tearing up. "I want you all to understand that we will get through this together. … Love will prevail."

Many educators nationally say they feel unsafe, and a survey by the panic-button provider for Barrow County schools said nearly 60% of teachers and administrators across the country had seen colleagues quit over safety concerns.