After summer break, an "upgraded" Arnold is thrilled to be back in Ms. Frizzle's class. He is delighted to see nothing has changed, except that his old friend Phoebe has went back to her old school and replacing her is Jyoti Kaur.
Arnold and the other students in Ms. Frizzle's science class give her a tour and explain how the "ecosystem" of their classroom is- perfectly balance, nothing to change it.
Soon a woman with red hair arrives and knows everybody's names, what they did over the summer, and their hobbies. Arnold is startled until Dorothy Ann explains that it's a totally different Frizzle. Arnold is very scared and asks where his old teacher is, and then Valerie appears and explains that Fiona is her younger sister and will be taking over, while she and her tamarin companion Goldie have adventures of their own. ("Frizzle of the Future")