Students and parents alike were unnerved by the incident, saying the school has always been considered safe.
It was "overwhelming," said Julia Schoennagel, 14, a freshman. "It was my first day, and I was excited to meet my teachers and see who was in my classes," she said. "It was unreal, I couldn't believe it. You never think that would happen at your school."
Junior Ryan Brady, 15, said he was several feet away from the shooting victim and saw blood on his shirt. As the counselor was tackling the shooter, his gun went off again, the bullet hitting the ceiling, said Brady, who got under a table to hide.
"After I got outside, my hands were shaking really bad," he said. "First thing I did was call my mom and told her everything that happened, and told her I was all right."
Police officers were already at the school, standard procedure for the first day of classes. County Executive Kevin Kamenetz said police and school officials responded in "terrific fashion" to the shooting.
"Obviously it's a difficult world we live in today, and I kind of hoped that Baltimore County would be immune from this type of activity," Kamenetz said. "But apparently we're not."
The shooting prompted swift reaction from elected officials, including Gov. Martin O'Malley, who said he had spoken with Kamenetz about the incident. "It takes all of us working together to make our schools safer for our children," O'Malley said.
Students were escorted to the nearby Perry Hall Shopping Center at the corner of Ebenezer and Belair roads, where parents were instructed to meet them, police said. Hundreds of visibly shaken parents and others gathered at the shopping center as police helicopters hovered overhead.
Students like DiPaula were able to text or call parents, siblings and friends. At least initially, incomplete information led to some confused and frightening moments.
"He pointed it at me but I'm OK," DiPaula texted his father, John DiPaula, who was at work and had no idea what his son meant.
A co-worker told the elder DiPaula about the shooting, which sent him into a panic because then he couldn't reach his son. Nick DiPaula told him later that he was unable to text or call again because teachers were taking away students' cellphones.
"I thought that was the most ridiculous thing," John DiPaula said. "He's afraid to call me because they're going to take his phone away? I mean, when is it appropriate to use your phone in school? Can you think of any better time than today?"
Nick DiPaula's sister, senior Julianna DiPaula, was in English class when a vice principal's voice came over the loudspeaker and said, "Code red, code red, we're under lockdown," she said.
At first she and her classmates thought it was a drill, she said, but then students started getting text messages saying there had been a shooting in the cafeteria.
"Then it hit me. Oh, my God, my brother is in that lunch," said the 17-year-old.
More than an hour later, Julianna and Nick were able to leave the school and meet their father in a nearby parking lot. When she saw her brother, Julianna ran to him, she said. "As soon as I saw him, I just started crying and hugging him," she said.
Miranda Wienecke, a junior, said she saw the suspect with a "huge black thing" that she soon realized was a gun.
"I saw people getting under the table," Wienecke said. "Then I saw people running. We heard this huge boom, then there was another one, everything happened very fast."
Senior Arielle Brown said students were in the cafeteria for about 15 minutes when they heard a pop, a sound she compared to the opening of a bag of chips. She and Wienecke said Wasmer and another teacher intervened, putting the shooter in a bear hug and pinning him against a nearby vending machine.