In the lobby at P.S. 28 in Corona, Principal Robert Quintana constantly greets his young students as they walk by.

The school is a warm, welcoming place, serving 3-K through second grade students, but in September an agitated adult intruder got inside and refused to leave.

“He tried to go up the stairs over here,” Quintana told NY1. “And the agent, the school safety agent, stood in front of him and he pushed her down on the steps. And then instead of leaving out the front door, he then went into the left elevator.”

The elevator was out of service, and staff ran to get Quintana.

“He got physical with me. So things became very physical and fortunately I was able to restrain him,” Quintana said.

Working in a school serving very young children, Quintana says all he could think about was protecting the kids.

“I was—I was frightened. I didn't know if he had a weapon or not. I never could have predicted anything like that in my life,” he said.

Within days, the DOE installed a locking front door system—one that New York City Public School Security Director Mark Rampersant says the city will put in every public school, starting with those serving the youngest children this fall.

“We want parents to know that we are in lockstep with them with their expectation to keep their child safe and send them home the same way we drop them off, just a little bit smarter,” Rampersant said.

It will represent a fundamental change for how parents interact with schools: They’ll no longer be able to walk right in. Quintana showed NY1 how the system works.

“So everything now is locked, which is really good. The only door that's remote access is this left door. All the signage is here in multiple languages,” Quintana said.

Visitors hit a button, and inside, school safety agents can see them on a small tablet, and ask them their reason for visiting. If acceptable, they're buzzed in.

Once inside, they show ID, and sign in.

“We've been seeing nothing but positive from the parents because they like to know that when my child is in there, not just anybody can get access to them,” Quintana said.

But, when a city panel approved the contract for the system, some parents worried it would not stop someone who belongs in the building from doing harm and could create a false sense of security, especially in large schools, where agents may not recognize everyone they buzz in.

“There is no 100% foolproof system. The one thing that I can assure you is that every one of our schools has an amazing school safety agent at the door,” Rampersant said.

The agents will be trained to recognize suspicious behavior and ensure people who need access, get it.

“This is not a system that's meant to keep our parents out of our schools,” Rampersant said.

For Quintana, it’s a system he believes would have prevented September's incident. But he says he’ll step up again, if he has to.

“I get choked up. Parents drop them off and they expect me to protect them. How do I not, right?” he asked.

He says it’s still hard to think about, but a little easier with the added security of a locked door.

“It's hard for me to just stay composed sometimes, in full transparency, but I'm the luckiest person in the world. The luckiest principal,” he said.