No Messin' With My Lesson (Katie Kazoo, Switcheroo No. 11)

Katie's teacher Mrs. Derkman is a finalist in the Teacher of the Year contest, and a contest official is coming to observe her teach class 3A.  But when Katie suddenly turns into Mrs.Derkman, the teacher's chances don't look so good.  How can Katie control a class full of out-of-control third-graders when she's just a third-grader herself?