The 18-year-old did not leave the odd-looking package, according to Lt. Doug Mullendore of the Washington County Sheriff's Department.
At about 10:30 a.m., a teacher saw the package on top of a chair lift in the hallway between the gymnasium and cafeteria, Mullendore said. He described the package as a plastic bag containing wet paper towels wrapped around a firm object.
A resident deputy summoned a bomb-sniffing dog from Frederick. The dog indicated the package could have been an explosive device, Mullendore said.
School administrators held a fire drill, leading students outside into a nearby playing field. The state Fire Marshal's Office bomb squad arrived and Maryland State Police flew a bomb technician from Bel Air by helicopter.
Using a robot with a high-pressure water cannon, the bomb technicians opened the package and exposed its contents. It was not a bomb or any type of explosive device, according to Mullendore.
The Sheriff's Department removed the remains, including a white powdery residue, which will be sent to the laboratory for analysis.
"There was never any danger," Principal Helen Becker said.
"We just wanted to make sure everything was as safe as it could be," said Director of Secondary Education Boyd Michael.
Students went back into the school at about 1:15 p.m. and Becker assembled them for a discussion in the gymnasium. Classes resumed and smiling, laughing students appeared to take the disruption in stride.
But news of the scare had spread in the community and dozens of parents arrived to take their children home. At 1 p.m., crowds of parents stood on the hill overlooking the field as many waited to sign their children out.
According to Michael, about 40 percent of the students left. It was the first time a Washington County school has been evacuated since the Littleton, Colo. shooting April 20, Boyd said.
Heather Lewis, an eleventh-grader, said she was frightened about the package and called her mother on a cellular phone. "This is a mess," said Carla Lewis, who came to retrieve her daughter.
Karen Harr said she had a feeling something was wrong and drove through the school complex Tuesday morning. "I saw everyone in the football field and I knew something was up," she said.
Harr took home her son, Clayton, a senior. "It's really scary for a parent," she said. "You can say it's just rumors but you don't know for sure. I'm not one of those people who thinks it can't happen here."
Harr said she appreciated the school's precautions. "I'm just glad they're all out and safe," she said, looking down at the field.
Broadcast reports that high school students had been dismissed were untrue, according to Washington County Board of Education spokeswoman Donna Messina.
Dismissal occurred at the normal time and it was orderly, according to Becker. She praised the students and parents for their calm. "The parents have been understanding and the kids have been perfect," she said.
-Staff Writer Marlo Barnhart contributed to this article.