I have strong memories of reading a chapter book in my grade school library where Encyclopedia Brown (or maybe another boy genius) plays harmless pranks on this hometown with the cooperation of the other kids. I remember it as a special edition of Encyclopedia Brown outside of the series where he solves mysteries.
In one story, the kids take all of their change from purchases as pennies for a few weeks, building up a huge stockpile of pennies — and then on a predetermined day, they go on a spending spree all over town making payment with pennies, leading to overflowing cash registers and befuddled clerks. In another story, the kids
In another story, the boy genius bets a friend that he can fool the whole town into believing that an imaginary person exists, and manages to get a library card and open a bank account in that fictional person’s name. He also describes the appearance of that person enough times that he convinces a whole crowd that they have seen him. He eventually reveals his duplicity and everyone is shocked to learn that they were fooled.
I have done hundreds of keyword searches on Google to track down this book, and have completely failed. I hope you can help me!