The students at the school in Edmond, Oklahoma, clapped and chanted “U-S-A” for Cuban-born Yanet Lopez to celebrate her achievement.
Ms Lopez, who works at Prairie Vale Elementary School in the city, was applauded and hugged as she walked through the school’s corridors on a victory lap.
In video of the heartwarming scene, Ms Lopez carried a miniature American flag and held her hand over her heart.
“Every morning Ms Yanet walks into the office and proclaims ‘Good Morning, Pretty Ladies’ and with a smile and a laugh she always starts the day with a positive message,” principal Michelle Anderson, wrote on the school’s Facebook page.
“We laughed that she would know more than those of us born in the US. I am honoured to work with one best US citizens I know,” she added.
Applicants for US citizenship have to take an oral test on US civics and history before they can become naturalised citizens.
There are 100 questions on the country’s system of government and other topics, and they must get at least six out of 10 correct at their final interview.
Once they pass the test they can then attend an oath swearing ceremony to become full citizens.