Bomb has been planted!
This is a Surprised Pikachu meme. This is an unusual example of the meme, because usually these memes are used to poke fun at people that do stupid things and are surprised at the consequences, however in this one, the people that are surprised are probably actually just scared that their classroom is about to be blown up because they misunderstood the message that was just broadcast from the kids phone.
Seeing someone scared can be funny because fear is a negative emotion. Basically this boils down to laughing at the misfortune of others.