It's not a series, exactly, but there are two are three: Sideways Stories from Wayside School. There's an overall plot, sort of, but each chapter is an individual story about an individual person in the classroom. The reading level is a little high or a little low, depending on what measure you use (
http://www.scholastic.com/teachers/book/sideways-stories-wayside-school), but my four year-old has been enjoying them. He doesn't exactly read a whole chapter at a time independently (hello, attention span), but we'll go back and forth by paragraph, or he'll just sort of pick it up and start reading at random. The way the chapters are organized around characters make it easy to just read at random.
