The Magic School Bus Inside the Human Body by Joanna Cole, 1989.

Ms. Frizzle’s class is now studying the human body, and they are going to take a trip to the science museum to see an exhibit about how bodies digest food for energy.  Of course, this is not going to be a normal field trip, but it is weirder than anything the students could have expected.

After the class stops at a park for lunch, Arnold is accidentally left behind, and before they can go back for him, the bus shrinks small and is accidentally swallowed by Arnold.

While his classmates take a trip through Arnold’s body, Arnold panics at being left behind and makes his way back to school by himself, accompanied by a sympathetic bird, not knowing where the rest of the class went.

The book ends with a section of true and false questions that point out the impossibilities of the class’s field trip inside a student’s body, but also points out that the information about the human body itself is true.

The book is currently available online through Internet Archive.