The Magic School Bus - Under Construction - Ep. 29

The class is going on a field trip to the new suspension bridge currently being constructed. However, there is a problem: Wanda's mother, Mrs. Li (Rosalind Chao) needs her to watch her little brother, William, until she gets off her business call. The class decides to watch him together. He accidentally uses the Porta-Shrinker to shrink them (without Liz) and the bus and locks them in the bathroom. They use the materials available to them to build structures to escape. This is the first episode in which the bus does not transform in any way.