Mr. Sweeney was Sophie's high school senior teacher. He is in the beginning of Book 1: Keeper of the Lost Cities before Sophie moves to the Lost Cities. He had taken Sophie's class to the Natural History Museum in Balboa Park where Sophie met Fitz for the first time.
At the beginning of Book 1: Keeper of the Lost Cities, Mr. Sweeney had caught Sophie listening to her iPod instead of paying attention to the lecture about dinosaurs. He made her answer a question about how the Lambeosaurus differs from the other dinosaurs the class had studied. Sophie answered with ease, thanks to her photographic memory and the information card she had glanced at when she had entered the museum. After that, Mr. Sweeney mumbles something that sounded like "know-it-all" to Sophie. Some kids tease her for being a "superfreak", but she ignores them (as usual).