Topic: Kitchen Chemistry. It's Ms. Frizzle's birthday, and a field trip to the bakery seems perfect -- until the bus' shrinkerscope goes on the fritz and the class gets baked in a cake. With Dom DeLuise as the Baker.
It's Ms. Frizzle's birthday and the class planned a surprise party for her, but she's outside fixing the bus. When she comes in, she tells the class that their field trip to the bakery is canceled. Fortunately, repairs or no repairs, she gets a change of heart and plans it after all. The bus and class get baked in a cake while learning about kitchen chemistry there.