Police said he hadn't been in serious trouble before and had easily concealed the tiny gun on his bus ride to school and during his first class.
Johnson said police are going to charge the boy as a juvenile, in part because the charges he is likely to face do not carry a life term in prison. Juveniles charged with crimes are not identified.
Gladden, the Perry Hall student who allegedly shot his classmate, was charged as an adult with attempted murder. Borowy was released from Maryland Shock Trauma Center on Monday after undergoing multiple surgeries.
Tuesday's incident disrupted the Stemmers Run school day for many students, though the school was never closed.
Police first responded about 9:50 a.m. to find the school in lockdown and the boy who had pulled the gun — he also had a 6- to 7-inch hunting knife on him — under arrest by the school's resource officer, who was down the hall when the female teacher called for help on a hand-held radio. Dance said all county teachers are issued radios.
The school remained in lockdown until 11 a.m. as police searched it.
School officials called the parents of all of the estimated 25 to 27 students in the classroom when the gun was drawn, and those students and their teacher were all offered counseling almost immediately, Dance said. Principal Gordon Webb also sent a letter home to all parents, stating that counselors are on hand for all of the school's 600 or so students.
Shortly after the morning incident, many students remained in their classrooms as parents were invited into the school auditorium to be briefed on the situation. Some parents said they were happy with how the school handled the incident. Others expressed anger and frustration, saying they were left in the dark for hours.
Tom Van Blargan, whose 13-year-old daughter is an eighth-grader at the school, said he waited for two hours at the school with little information.
"They're telling us everything is fine, but [my friend's daughter] was texting her mom saying there were police with bulletproof vests in the hallways," Van Blargan said.
Chasity Slaski of Hawthorne said she "freaked out" after receiving an automated message from the school alerting her to an incident but providing no details.
Slaski, whose 12-year-old son, Michael, is in seventh grade, said she jumped in her car and drove to the school, and plans to keep her son out of school Wednesday.
"I will move my whole life to protect my kids," Slaski said. "I'm not willing to jeopardize my kids' safety."
Her son said his class wasn't told why they had to stay in the same classroom through multiple periods.
"They didn't tell us what happened, but we've been practicing lockdown all year," he said.
Many students left the school with their parents before the close of the day, and many buses left the school almost entirely empty just after 3 p.m.
Johnson, Kamenetz and Dance all implored parents to ensure any weapons at home are secured.
"This is the responsibility of the parents and it's also the law," Kamenetz said.
State law requires gun owners to secure loaded firearms from children 15 and younger, and violations are misdemeanors punishable with up to $1,000 in fines. Gun locks cost as little as $1 online, Johnson said.
In the Perry Hall incident, police said Gladden got the long gun he allegedly used to shoot Borowy from his father's house. Gladden is 15, but police have filed no charges against his father.
Apart from their focus on gun safety, officials also roundly praised the unidentified Stemmers Run teacher for her actions.
"Again, one of our teachers acted heroically," Kamenetz said, in a nod to Wasmer's actions at Perry Hall last month.
Beytin said teachers by nature think of their students first. "It's like a parent. You can't live without knowing that you did everything you could do," she said.
But the teachers are victims of the violence, too, Beytin said, and the incidents have taken an emotional toll.
"After an incident like that, you realize what you've just been through," she said.
Baltimore Sun reporters Yvonne Wenger, Jessica Anderson and Erica L. Green contributed to this article.