Scholastic's The Magic School Busmeets the Rot Squad
Something's rotten in Ms. Frizzle's class! It's "Accidental Science Project" day. The kids have to bring in the moldiest things they can find in the refrigerators. Everybody agress that mold is just disgusting! But when they go on a field trip inside a rotting log, they discover that all that dead-looking stuff is actually alive.