Those are the kids from Deaf Film Camp at upstate New York's Camp Mark Seven performing the song in sign language.
There have been plenty of fun video covers of the song – the one from the New Zealand senior citizens was particularly inspired – but this one has to rank among the most infectious.
It should be no surprise that it's such a professional production. Deaf Film Camp, launched last year, teaches screenwriting, directing, acting, effects and cinematography to deaf campers.
"We're breaking new ground with the camp and for these campers, who see – and can express – the world in unique ways," its director, Stacy Marie Lawrence, told The Hollywood Reporter.
On Thursday, Pharrell caught wind of the video, and Tweeted a message of thanks to the camp before posting the word 'Happy,' spelled out in sign language, to Instagram.