According to investigators, the girl told them it was a "prank" committed with a friend at her school and she never had any intent to harm anyone.
The case is being forwarded to the District Attorney's Office, police said.
Authorities said this investigation continues.
There were at least two accounts on which threats were posted. It wasn't immediately known if the teen was behind both accounts.
Parents were alarmed after learning several of the threats specifically mentioned Philadelphia schools.
"That's disgusting. That's very scary," said Cathy Everett, of South Philadelphia.
"I think they're very sick people," said Teresa Anastasio, of South Philadelphia.
Those are some of the responses Action News received about an Instagram post made through an account called Clown_of_Philly.
The post, which appeared Saturday, warns of a shooting attack at a local school.
It states, "Coming to Neuman Goretti on Monday!! Going to shoot all the students and teachers."
By Sunday night, word had spread. Action News sampled reaction at an Italian festival taking place a few blocks from Neumann Goretti.
"I'm a little nervous. Once again, it's such a close-knit community that we don't let that happen," said Pete Sapienza of South Philadelphia.
"Social media is getting out of hand now. We need to stop and think before we post stuff," said Anastasio.
Another post that was gaining momentum Sunday was from an account called phillyclown.215.
It threatened students at Northeast Charter High School.
The post stated, "We coming to all north east charter high schools.... we will be after your let out on Wednesday."
A second post from same the account says, "the gang is in philly..." and "ya'll ain't safe. it's 12 of us yaheard."
But late Sunday or early Monday that account user posted again, saying "the clown joke is officially over."
To school officials and police, none of this has been a joke.
In a joint statement released Sunday, the Philadelphia School District and Philadelphia Police Department said:
The School District of Philadelphia's Office of School Safety alerted both the Philadelphia Police Department and the Office of Homeland Security Saturday upon learning of these disturbing social media posts. The safety of our children and the communities surrounding our schools is a top priority for us and we are taking this matter seriously.
The Philadelphia Police Department is actively investigating these social media posts and pursuing those responsible for them. The SDP Office of School Safety will continue working in concert with the PPD to ensure the safety of our children and our schools when school reopens on Wednesday after the holiday break.
While there have been similar social media posts regarding schools in other states and in Pennsylvania the PPD and the SDP remain in contact with Homeland Security and will continue to investigate local threats on social media.
Some schools within the Philadelphia Archdiocese have also been threatened.
We received a statement from a spokesperson on the matter that reads in part:
"Schools that were specifically named in threats have been in direct contact with law enforcement and will have a police presence tomorrow as a cautionary measure."
In the meantime, neighbors in South Philadelphia say they'll be keeping an eye out.
"The neighborhood would never let it happen because it's such a close and tight community. It would definitely be stopped," said Sapienza.
Anyone with information surrounding these posts should contact the Philadelphia Police Department at 215-686-TIPS (8477).
Last week, the Toms River Police Department said security is being stepped up at schools in their area due to reports of clown sightings.
The department offered some background into the clown sightings.
As some of you may be aware there is a story circulating around the nation regarding "creepy clown sightings." The initial reports seem to stem from an organized movie promotion and have now morphed into copycat reports throughout New Jersey. The recent social media posts and threats to schools seem to originate from outside of New Jersey. Facebook shares and retweets have brought these concerns to New Jersey.
In Schuylkill County, Pottsville police received a report of two men, dressed as clowns yelling at children at a playground on Monday, and then took off in a vehicle.
In South Carolina, police increased patrols after new reports of a person dressed as a clown trying to lure children into the woods in South Carolina.
Three children at an apartment complex told police the clowns tried to attract them with money in Greenville County.
The week prior, a mother in South Carolina called police after her son, and others around a different apartment building, claimed they saw clowns behind a basketball court.
"I thought my child was seeing things, and then the next day I had about 30 kids come up to me and say 'did you see the clown in the woods?' " said Donna Arnold.
A teenager in Coffee County, Tennessee, claimed to be attacked by a clown in recent days. The Tennessee Highway Patrol issued a warning to residents about clowns trying to lure children into the woods.
In Reading, Pennsylvania, police say someone in a clown mask was involved in a teenager's killing.
Police say they're sorting through various accounts of the incident that include the victim wearing the mask, the stabber wearing the mask and someone else wearing it.
In nearby Martinsburg, Blair County, police have charged an 18-year-old man with harassing, threatening and prowling around his neighbors' home while wearing a clown mask and holding an Airsoft toy pistol.