A quiet joy filled teachers and staff at Levittown’s Wisdom Lane Middle School on Monday morning as eighth-grader Jessica Lemus walked the halls with her friends again for the first time in nearly five months.

It was a welcome sight for many who remembered her being taken out on a stretcher only three weeks into the school year, they said.

Lemus, now 13, collapsed suddenly during science class on Sept. 16, suffering a cardiac arrest. Harrowing moments followed, with several teachers giving her CPR on the classroom floor while students were sent into the hall. The school nurse used an automated external defibrillator, or AED, to revive her.

Rushed by ambulance to Nassau University Medical Center in East Meadow, Lemus eventually had open-heart surgery in Boston and recovered at home for months.

“I couldn’t thank them enough,” Lemus said Monday morning in guidance counselor Christine Elder’s office. She had hugged several teachers, chatted with friends near her locker and made many in the school community smile throughout the day.

Jessica Lemus, 13, who collapsed in class on Sept. 16 and was revived by teachers and staff before being rushed to the hospital, returned to Wisdom Lane Middle School in Levittown on Monday, Feb. 1, 2016. Photo Credit: Howard Schnapp

“She is so courageous for being here after having gone through all of that,” said Elder, who gave the girl a bracelet that reads “strong, beautiful and brave.”

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Although Lemus had been born with a small hole in her heart and had been seeing a cardiologist regularly for a heart murmur, there were no immediate warning signs on that morning in September. Lemus said she felt fine and went about her routine, riding the bus to school and walking from class to class.

At 11:10 a.m. on that day, science teacher Carole Going was taking attendance when co-teacher Ann Marie Carlson noticed that Lemus appeared weak.

She started to ask “Are you OK?” when Lemus, who was sitting on a tall stool, pitched backward, hitting her head on a table and falling onto the floor.

Going asked all the other students to leave the classroom and ran to get another teacher, Megan Olsen, a former lifeguard trained in cardiopulmonary resuscitation.

Olsen began performing chest compressions on the girl. In all her years as a lifeguard, she said, she never had used CPR. But her training kicked in as she worked to revive Lemus.

“I went home that night, and asked my husband, ‘What if I did it wrong?’ ” she said. “And when I heard she was OK and she was speaking, it was like a great weight was lifted.”

As Olsen performed CPR, another staffer alerted health teacher Jordan Dasch, who is a certified instructor in CPR and the use of AEDs, electronic devices that can restore a regular heart rhythm during sudden cardiac arrest.

“This was the first time I had to use these skills in a real situation,” said Dasch, who did not have Lemus as a student. He also performed CPR.

The school nurse, Carol Fitzpatrick, was called to the classroom and used the AED on Lemus, who finally took a breath.

“I still get very emotional when I talk about it,” Going said. “It was unbelievable — the image of her being shocked by the AED and her taking that first breath and you could see her gasp.”

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Going rode with Lemus in the ambulance to NUMC, talking to her the entire time and reassuring her she would survive.

“Mrs. Going — she is one of the most amazing persons,” said Lemus’ mother, Reina.

From there, the girl was taken to Cohen Children’s Medical Center in New Hyde Park, where it was discovered she had two irregular heart valves and an enlarged right chamber.

Lemus was airlifted to Boston Children’s Hospital. She underwent open-heart surgery to repair the valves and implant an internal defibrillator, which could prevent sudden death from an irregular heartbeat.

Going visited her student at Cohen Children’s, drove twice to see her in Boston, and continued to visit and call and text with her over the months. Going said she felt as if she’d become a part of Lemus’ family over the last several months, enjoying traditional Salvadoran dinners with them at their home.

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“Whether she likes it or not, I’m a part of her life for the rest of her life,” Going said. The teacher said she often thinks of how everything clicked that day: The staff in her particular hallway all knew CPR, and the AED was easily accessible.

“All these little things that lined up perfectly,” she said.

Both Olsen and Going gave credit to the Louis J. Acompora Memorial Foundation, formed in memory of the 14-year-old Northport athlete who died March 25, 2000, as a result of sudden cardiac arrest during his first high school lacrosse game.

In 2002, Louis’ Law was enacted, mandating that schools have AEDs on-site, including at athletic events, as well as people trained to use them.

Olsen and other staff attended an American Heart Association dinner in Woodbury on Sept. 23 and learned that the Acompora Foundation counts Lemus as the 88th “save” statewide as a result of the passage of Louis’ Law.

The Levittown district also offered CPR training for its entire staff soon after Lemus’ collapse in September.

Monday, Lemus returned to the fifth-period science class, sitting at her desk, opening her aqua-colored binder and listening to Going’s lecture on erosion. The class carried on smoothly.

Many of Wisdom Lane’s longtime teachers — even principal John Avena, an educator on Long Island for 25 years — said they had never experienced an event quite like what happened with Lemus.

“I will relive that day over and over in my mind,” Going said. “And that’s OK, because she’s here and she’s fine.”