According to a police report, other students overheard the boy saying he had a gun, but no firearm was found.
The student did not threaten anyone with the knife, said North High School Principal David F. Reeder.
Weapons are barred from school property. The student was suspended pending a conference with his parents and school officials.
Two students from the Alternative School were arrested Friday after reportedly making threats while on a school bus that morning, according to school system spokeswoman Donna Messina.
The students also were suspended from school for three days, Messina said.
In an effort to ease fears and restore calm to the school system, parents, employees from the school system's central office and retired school system employees will be joining local law enforcement officers at Washington County schools on Monday.
More than 50 current and retired school system employees are expected to spend all or part of their day at a school, Messina said.
A half dozen parents are expected at North Hagerstown High School on Monday, where they will watch the exterior doors, Reeder said.
"Hopefully this will calm the waters," said Reeder, whose school was still feeling the effects of rumors and fears Friday. About 14 percent of North High's students were absent Friday, which is more than twice the normal absentee rate, Reeder said.
Also, Maryland State Police, Washington County Sheriff's deputies and Hagerstown City Police will be at every middle and high school in the county on Monday.
"These steps are a safeguard," Messina said. "Not because of any particular cases. We hope these extra steps will give parents some calm."
School officials have also arranged for a video of a Monday panel discussion on school safety to be broadcast on Antietam Cable.
The hour-long video will appear on cable channel 6 from 11 a.m. to 3 p.m. both today and Sunday.
The video features short speeches by school officials, law enforcement officers, and parent and student representatives talking about school safety.
"The video is just one more resource available to us. One more way for us to reach the community," Messina said.
And at Bester Elementary School, the principal is trying to turn the nervous and fearful mood of the school around with a Monday breakfast get-together for the students.
"We have kids living in fear and kids crying. We want them to celebrate the good things," Bester Principal Drenna Reinick said.
"I told a student, 'You have to stop worrying about what could happen. I could walk out in the street and get hit by a car but I'm not going to stop walking because of that possibility, a remote possibility,'" Reinick said.
These actions are in additional to previous measures, including the starting of a school system hotline, for parents or students to leave messages.
Since the hotline began taking calls Wednesday about 45 people have called it, Messina said.
"We're trying to make sure people not only feel comfortable, but that we do the things we need to do for school safety. Schools are still the safest place for a kid to be," said Schools Superintendent Herman G. Bartlett Jr.
"This is an unusual time period and we understand the stress and we're trying to do everything we can."
"Rumors and actions are being taken very seriously. We are reacting to these threats very seriously."
The three students arrested on Friday were the latest alleged school-age offenders in the Tri-State area to be singled out by authorities since the Colorado shootings:
-- A 16-year-old student accused of threatening to blow up Washington County's Evening High School was ordered detained Friday at the Noyes Center in Rockville, Md.
The teen, who disputed a school secretary's version of alleged threats made Wednesday, will remain there until a violation of probation hearing on an unrelated matter is set for him in the near future, according to Washington County Circuit Judge John H. McDowell. Evening high school classes are held at E. Russell Hicks on the Sharpsburg Pike.