The Woodbury City Police Department told Action News that the juvenile who made the original threat on Sunday evening is a student from Woodbury.
Police do not believe the 12-year-old had any access to weapons and described this as a "mental health issue" and "a cry for help."
Three other juveniles have been arrested in Glassboro, Harrison Twp. and Washington Twp.
At this time, police do not believe the young people knew each other. The other three arrested were described as copycats who, in some cases, modified the original post to mention other schools.
One of the other suspects is 15 years old. Ages were not released for the remaining two.
Threats were made against Woodbury City Public Schools, the Deptford Township School District, the Haddon Heights School District and Glassboro Public Schools.
Schools in the Woodbury and Deptford school districts were closed on Monday.
Woodbury Schools will be open Tuesday, according to a letter from the superintendent.
Leaders with Woodbury City Public Schools said that while they did not believe the threat was credible, schools would be closed as a precaution.
"We are working closely with the Woodbury City Police and Chief Ryan to allow them the time needed to investigate and track the source of this threat thoroughly, and again, there is no immediate indication that the threat is valid," the district wrote in an online announcement.
The Deptford Township School District also said the decision to close was made out of an abundance of caution.
Transportation to GCIT and other out-of-district schools will not be provided, the district said, and all extracurricular activities have been canceled.
"We will continue to work with law enforcement officials to determine our next course of action and to ensure the safety of our schools and all of our Spartans," the district said in an online statement.
Parents and guardians were on edge, especially after last week's deadly school shooting at Apalachee High School in Georgia.
"I hope it's an opportunity for parents to have a discussion with their kids about schools and shootings and social media," said Doris Williams, whose two grandsons attend Holy Angels Catholic School in Woodbury, which was mentioned in the original threatening post.
Anyone with information on this incident is asked to contact the police at 609-221-6721.