Every year there is one Friday when there are no classes and everyone at Culver Creek is expected to report to the gym for Speaker Day, a notoriously boring day—part of the prank is to "shake Speaker Day up a bit" (84after.1).
The task is to persuade the Eagle that the junior speaker is a doctor and scholar of "deviant sexuality in adolescents" (84after.2); this is where Miles's dad comes in.
So Miles calls him and tells him that he's got to play a part in the new greatest prank of Culver Creek. And Miles's dad says he'll do it but Miles can't ever tell his mother. Way to have a cool dad, Miles.
The prank won't work without the help of the Weekday Warriors (an idea that the Colonel hates), but the Warriors approve wholeheartedly. This must be a pretty great prank if sworn enemies are coming together to pull it off…
The leader of the Warriors and Miles go to talk to the Eagle, telling him their choice in speaker. The Eagle calls the speaker (who is actually Miles's father) and approves of the choice.