It seems to go back to the United Farm Workers. Here’s the Wikipedia entry (kind of funny):
“Yes We Can” is a collage-style music video inspired by US President Barack Obama’s popularization of the slogan “Yes we can.” During the 2008 US presidential campaign following the New Hampshire Democratic Primary, then-Senator (now President) Barack Obama, used the English translation of the United Farm Workers slogan “Sí, se puede” (Spanish for “Yes, it can be done”[1]) in his speech. The slogan dates back to 1972 when Cesar Chavez and the UFW’s co-founder, Dolores Huerta, came up with the slogan during Chavez’s 24 day fast in Phoenix. Arizona.[1]
The song was released on February 2, 2008, by the Black Eyed Peas member will.i.am on Dipdive.com and also on YouTube.com under the username ‘WeCan08’.[1] It was honored with the first-ever Emmy Award for Best New Approaches in Daytime Entertainment.[2] The term “Yes We Can!” became a secondary slogan for Obama’s campaign.
I think the girls in my 3rd grade class started this, when we asked a group of boys on the yard playing tetherball..
Can we play too? they said...Girls don’t know how to play tetherball and we said *Yes We Can”*...:)