The changes include alarming every door to the school, and securing windows and other points of entry that could allow someone to slip in unnoticed.
"From now on, we will know where and how people enter and exit our school," Holiday said.
Another major change, according to Holiday, is an overhaul of the school's guest policy. Previously, parents and certain alumni mentors weren't subject to the same security measures as students, which include a walk-through a metal detector at the school's main entrance.
Donte Walker, 18, was one of those alumni asked to return to the school to speak with his peers, Holiday said.
"He was a mentor. He went to college and played football there," Holiday said. "This particular individual took a wrong turn somewhere, and, unfortunately, this incident happened."
Police say Walker provided the gun used to shoot a 17-year-old student and his girlfriend, also a student, in the school's gym.
According to Holiday, Walker signed in as a guest the morning of the shooting, spoke with a few classes and then left at midday. He returned just before dismissal, allegedly with the gun, through an unlocked patio door behind the school.
"With that program, there was a level of trust and maybe complacency, though I hate to use that word," Holiday said. "Before the incident, we never felt these guests needed to be searched. Now, we do."
Holiday stressed the incident was "isolated" - the only example of gun violence at the charter, which is attended by nearly 700 students from throughout the city.
He believes that Raisheem Rochwell, 17, who police say fired the gun, sought the firearm after being attacked on his way home by kids not affiliated with the school.
"Our city is our city. The streets outside here can be unsafe," Holiday said.
"I'm confident our students know we're doing everything we can to make this building a safe place."
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