This all happened around 9:40 a.m. at the school in the 200 block of Walnut Street, inside a health class with at least 18 students.
Some parents took their children out of school early after learning of the incident.
Superintendent Dr. Patrick Michel says it was the result of one student being bullied by another.
"Bullying continues to rear its head, and it easily could have been a situation where a kid could have been murdered," said Dr. Michel.
The school was locked down for about half an hour.
The armed 17-year-old student fled north on Walnut Street and was spotted and arrested by Salem police.
Officers also found a 35 caliber handgun with at least one round in it discarded in a nearby wooded area.
Parent Dorothy Derrickson says she's upset that the school has no working metal detectors.
"I'm worried about my kid's safety," Derrickson said.
The student is facing several weapons charges, including unlawful possession of a weapon on school property. He's also charged with aggravated assault.
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