Wayside School Gets a Little Stranger by Louis Sachar

The fifth book in the popular Wayside School series (or the third book, if one doesn't count the two focused solely on arithmetic). Wayside School Gets a Little Stranger picks up where the second book left off, with everyone's favorite school opening back up after the great cow incident (don't ask!). Full of thirty more short stories, the book is sure to reduce anyone to laughter, and is easily readable for those in their upper elementary years.