WATCH: Students Organize Moving Performance for Chicago Choir Teacher Returning to Classroom After Battle With Cancer

When Gabrielyn Watson, known to her students as Ms. Watson, returned to school after taking several months off amid a battle with cancer, she never expected what would be waiting for her.

The choir director at Chicago's Morgan Park High School took seven months off from teaching to fight multiple myeloma and was welcomed back to school this month with a moving performance of "Amazing Grace" by a throng of former students, including "American Idol" finalist Peter Frank.

Frank led off the surprise, standing in the hallway alone as he began to sing. Watson collapsed, overwhelmed with emotion as other former students join Frank in harmony. Soon, even Watson herself began to sing along.

The heartwarming homecoming was filmed as part of the "Someone Needs One" program by Kleenex. Frank, now 29, said he arranged the surprise to show his appreciation for "the most influential teacher he’s ever had."

Since video of the touching surprise was posted to social media last week, it has been shared more than 130,000 times and received more than 7.3 million views.

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Frank, who appeared on Season 10 of "American Idol," credits Watson with his career as an aspiring entertainer and called her "an unsung hero who deserves recognition." He recalls a moment when Watson gave him a tux for a chorus competition because he couldn't afford one. 

In the video, an emotional Watson said she loves her students.

"I want them to have the best opportunities so that they can thrive," she said.

Morgan Park High School Principal Carolyn Epps called Watson a "true educator" and said there is "no greater tribute to a teacher."

"It was a real tearjerker," Epps said. "She was so surprised and so happy, so it really touched your heart."

Watson described the surprise as "such a rewarding, healing and inspiring experience," according to Kleenex.