Karmack

"A strange little creature enters the lives of three tough guys at Higgins Elementary. And from that point on, pranks come back like boomerangs, smacking the bullies right in their fifth grade butts! Sully, the leader of the gang, is the only one who can see this creature, which he names Karmack. Sully learns that the creature's job is to balance all the bad pranks the bullies have pulled over the years. Karmack warns that if this fails, "dreadful, awful doom" awaits them all. Sully soon realizes that he must save not only himself but his friends, who have no idea why they keep getting boomeranged by Karmack!" -- Amazon website.