But then he saw an injured boy clutching his wounded arm. He watched his fifth-period math teacher, Michael Landsberry, walk toward a student and fall to the ground.
"When he pulled a gun, we knew what happened," Ferro told The Associated Press on Tuesday.
Moments later, the eighth grader was cowered against a wall with some classmates, burying his face in his hands as a 12-year-old boy waved a semi-automatic handgun and threatened to shoot.
Ferro didn't know the boy, but said he and others in the frightened heap tried to talk him out of firing.
"You could hear the panic," Ferro said. "He left, thank God."
By the time police arrived, the 12-year-old gunman had killed Landsberry, wounded two boys and taken his own life.
There were no easy answers Tuesday about what prompted the boy - who police are not identifying out of respect for his family - to take a 9mm handgun from his home and open fire at school. Police said they've interviewed 20 or 30 witnesses and are looking into any prior connection the victims had with the shooter. The boy's parents, who could face charges in the case, are cooperating with the investigation.
"Everybody wants to know why - that's the big question," Sparks Deputy Police Chief Tom Miller said. "The answer is, we don't know right now."
At a news conference Tuesday, law enforcement and school officials again lauded the teacher's actions, which police say gave students time to run for safety.
"I cannot express enough appreciation for Mr. Landsberry," Washoe County School District Superintendent Pedro Martinez said about the 45-year-old ex-Marine. "He truly is a hero."
"His actions yesterday I believe saved the lives of many children," added Col. Jeffrey Burkett, commander at the Nevada Air Guard's 152nd Airlift Wing where Landsberry served.
Police said they believe the shooter at one point tried to enter the school but couldn't open the door because of emergency lockdown procedures.
After killing Landsberry, the boy fired at a second student, hitting him in the abdomen. He then shot himself in the head.
The two 12-year-old boys who were wounded are in stable condition and recovering.
Parents clung to their children at an evacuation center shortly after the shooting while the community struggled to make sense of the latest episode of schoolyard violence, which happened less than a year after the Newtown, Conn., massacre.
Sparks, just east of Reno, has a population of roughly 90,000.
Under Nevada law, it is illegal to allow anyone under 18 to handle a gun without supervision. The offense rises to a felony if there was substantial risk the child would use the firearm to commit a violent act. However, the law doesn't apply if the gun was stored securely or if the child obtained the weapon unlawfully.
Landsberry was married, had two stepdaughters and coached several youth sports. He also served once in Kuwait and two tours in Afghanistan with the Nevada National Guard and was well-known in the school community. He served in the Marine Corps from 1986-90 and was stationed in Camp Lejeune, N.C., and Okinawa, Japan, according to military records.
His bravery came as no surprise to his fellow airmen.
"That's who he was," Chief Master Sgt. James Ross told reporters at the air base in Reno on Tuesday.
Senior Master Sgt. Robert Garrett attended middle school with Landsberry in Reno before serving as his supervisor in recent years. He described him as a tough ex-Marine who taught with compassion and was known as a Batman fan and soccer fanatic who would do anything to help anyone.
"Every one of the people I have talked to just knew that Mike was in there," Garrett said. "He was the guy that would have jumped in there to stop the bullets from hitting other kids. And sure enough, it was."
Mayor Geno Martini praised the response from officers, who arrived at the scene within 3 minutes of the initial 911 calls to find the shooter dead.
"You can never plan for anything like this," Martini said Tuesday.
Students from the middle school and neighboring elementary school were evacuated to a high school, and all classes were canceled. The middle school will remain closed for the week, while an adjacent elementary school is set to reopen Wednesday. A candlelight vigil was planned Wednesday night outside the middle school and a multi-faith prayer vigil at a church in neighboring Reno.
Ferro's dad took the day off and the two went out to lunch together on Tuesday, a day Ferro thought might never come.
"I'm still a little shaken up, depressed and confused," Ferro said. "The whole time I was hoping Mr. L. was OK, we'd all get through it, it was a bad dream."
Associated Press writer Michelle Rindels in Las Vegas and news researcher Rhonda Shafner in New York City contributed to this report.