The Magic School Bus Season 3 Episode 4 - The Magic School Bus Under Construction Episode Title:The Magic School Bus Under Construction.Episode Description:The class is going on a field trip to the new suspension bridge they're building. However, there is a problem, Mrs. Li (Wanda's mother) needs Wanda to watch her little brother William until she gets off her business call. The class decided to watch William together. William, only a toddler, accidentally uses the Porta-shrinker to size down the entire class (without Liz) and the bus and locks the class in the bathroom. The class uses the materials available to them to build structures to escape. This is the first episode where the bus doesn't turn into anything.