Pranks are cruel. They are thought of and executed with the intent to be unkind to someone else, with complete disregard for how the other person will feel. The prank perpetrators in this particular stunt are boys, whose response is they are 'just kidding'. This is a typical response from males of all ages who engage in mean-spirited behavior, and probably one of the reasons why their actions don't get categorized as bullying. But pranks certainly are unkind, mean-spirited, cruel.
There is a complete disregard for others during a prank because the perpetrator is focused on achieving some kind of satisfaction from pulling off the prank and the result of upsetting the intended victims. This is most likely why the boys don't feel badly about their actions as that is the whole point of a prank: to hurt someone else. Just like that show Punked from earlier in the decade that pranked a lot of celebrities "in good fun". But how many of those celebrities cried, or we're frightened? Who was it fun for? Only those perpetrating the prank. Pranks are not only a form of