The incident happened on Tuesday.
Investigators say the school resource officer received a tip about the gun shortly after 7 a.m.
"Somebody who saw something and heard something - they said something. And because of that, we were fortunate yesterday," Colonial Schools Superintendent Jeffrey Menzer said Wednesday.
The officer and a school administrator located the student in class and found the gun after a search of the student's backpack.
The discovery leaves many wondering:
"Where does he get access to that?" said Marques Coleman of New Castle.
The student was taken into custody and charged with Possession of a Deadly Weapon by a Person Under 18, Carrying a Concealed Deadly Weapon and Possession of a Firearm in a Safe Recreation Zone.
The student is being held at the New Castle County Detention Center on $30,000 cash bail.
As far as security measures go, the Colonial District has not installed metal detectors or weapons detection systems, but says they are up for consideration.
The superintendent making a request of families: lock up your guns.
"If they come into school they come from somewhere. But if they are secure in the home, it's just one added layer to keeping our students safe," says Menzer.