Starting at a new school this year, 15-year-old Sergio Peralta had all the typical teenage reasons to be nervous.

He was also trying to keep a secret: a hand that was not fully formed.

"In the first days of school, I honestly felt like hiding my hand," he told CBS News, the BBC's US partner. "Like nobody would ever find out."

Instead, a teacher at his Tennessee high school learnt his secret and assigned his engineering class a project: build Sergio a new hand.

"You're supposed to be engineering, coming up with new ideas, solving issues," Hendersonville High School student Leslie Jaramillo told WTVF, a local CBS affiliate. "And just making things better than how they used to be."

Sergio, born with a right hand that did not fully form, had become used to it. He learnt to write with his left hand and said he could get by with nearly everything else, figuring out techniques for basic tasks like carrying his water bottle from class to class.

The suggestion from his classmates at his school in Hendersonville, 30 minutes northeast of Nashville, came as a happy surprise.

"They ended up offering me, like, 'We could build your prosthetic hand', and I never expected it," Sergio told CBS. "Like, never in a million years."

Jeff Wilkins' class, Engineering: Design and Development, was designed to take the theoretical "and turn it into reality", Henderson High School principal Bob Cotter told the BBC.

Sergio's robotic hand "is a testament to the students we have here who care about each other and the programme that Jeff Wilkins has built," he said.

The small class of students spent four weeks designing, 3D printing and sizing the prototype to Sergio's hand, and becoming friends in the process.

After a month of work, they put his hand to test with a game of catch, a hobby that was once out of reach.

"[After] living without a hand for 15 years and they actually offered me two is actually pretty cool," Sergio told WTVF. "Like changed my life"