The student, whose name is not being released, has been remanded into juvenile detention.
Police said a teacher noticed a smell of marijuana coming from the student, and that he was also applying heavy doses of cologne to mask the smell, and notified a vice principal. Parents were alerted to check their email for an important message.
"It just said they found the kid with a weapon in his backpack, and they were investigating an unrelated incident when they found it," said parent Mickey Spurgeon, who has a daughter attending Warren Woods.
The student was taken to a school office and searched.
Police said the vice principal found three wrapped packages of marijuana, a bag of ammunition and an unloaded .22-caliber revolver that was wrapped in a bandana.
Police said it appears the student was going to sell the drugs to other students.
The high school didn't have classes Friday because of the holiday.
"It's scary, I mean, I do school of choice here because it is a better school district, and it's scary when you hear that," said Spurgeon.