Actually, it isn't. Racism is based in the belief that one group is superior to the other. If one is making an ironic joke, it does not imply any intent of racism on their part.
Personally, I couldn't care less what people believe, I care what people do. And in this case what they did was clearly racist.
Racism is defined as prejudice, discrimination, or antagonism directed against someone of a different race based on the belief that one's own race is superior.
What these kids did was not racist.
Second, this was clearly antagonism directed against someone of a different race, and is therefore racist.
As for prison, I'm implying what consequences you would assume for individuals who would be so vocal about what you would call their racist acts.