B is for Bigfoot is a short story in The Dresden Files series by Jim Butcher. It is set between Fool Moon and Grave Peril. It can be found in Under My Hat: Tales From the Cauldron, and was later included in the anthologies Working for Bigfoot and Brief Cases.
Strength of a River in His Shoulders hires Harry Dresden to help his teenage son Irwin Pounder. When Dresden talks to Irwin's mother, Dr. Helena Pounder, she informs him that her son has been showing up with injuries but is refusing to tell her how he had been hurt; the likeliest guess is that he's being bullied by another student. She wants Dresden to find out who has been hurting him, and has arranged for him to stand in for the school janitor.
The next day, Dresden shows up at Irwin's prestigious private school. Over lunch, he watches as a couple of boys harass him at his lunch table and a teacher punishes Irwin with detention for the ensuing mess. Harry Dresden soon realizes that the bullies are the scions of an unspecified supernatural being, and the coach is their guardian. In a confrontation, Coach Vogon warns Dresden not to try to interfere with them.
During detention, Dresden gives a quick talk to Irwin about the need to fight back, but Irwin is unwilling to risk hurting others. The boys return and Irwin tries to talk them out of violence, but is ignored and quickly overpowered; his strength however is too much for the boys, which are forced to retire after a sample of it.