It happened Thursday just before 3 p.m. at Sonrise Christian Day School in Linwood, Pennsylvania.
The day care's director said dozens of children were inside at the time.
"The only time I got real nervous was when he said, 'I have a weapon,'" recalled Director Angela Ewald Sannino.
As the man was attempting to get inside, police say the day care's assistant director called 911 while Sannino placed the building on lockdown using her walkie-talkies.
Sannino told Action News she was simultaneously trying to keep the man calm.
"I cannot commend the staff enough. They kept everyone safe. Those doors were locked. Those children were quiet as can be. It was wonderful. I'm so proud of my staff right now," Sannino said.
Most of the incident was captured on video. Staff members said what the cameras didn't see, however, was what happened once police arrived.
They told Action News that when officers got to the scene, the man became agitated, stripped naked, ran up a set of stairs, and then tried to get into the building's main area on the top floor.
Fortunately, he couldn't get in because of the tight security.
When trying to escape, he allegedly ran into a closet and jumped from a window that was 25 to 30 feet from the ground.
"We didn't know what happened, down here all you heard was, 'Bang! Bang! Bang! Bang!'" Sannino said.
Police in Lower Chichester were able to take the man -- who was naked, bloodied, and injured -- into custody.
Saheir Sadler-Chandler, 18, now faces a slew of charges, including simple assault, institutional vandalism, public drunkenness, and more.
The investigation is ongoing, and day care staff say they are thankful teachers were ready to keep the children safe and for the police's quick response.
"I don't think he came with ill will or to do any harm, but you can't come into a school building," said Assistant Day Care Director Jaimee Bartholf.
"Later that day as I was driving home in my car, I was like, 'Ugh, that could've been really bad but we thank God that it wasn't,'" Bartholf added.