Long story short, it was discovered that there was over 40 other flying space rocks that were somewhat similar to Pluto. At least one of them was larger. Rather than making children memorize almost 50 planets, they decided to classify Pluto, and the others as dwarf planets.
Astronomers ruled on August 24, 2006 that Pluto is not a planet. It involved 424 astronomers who decided to declare Pluto as not part of the nine Planets. It was demoted to dwarf planets that are part of sea objects of others objects occupying the same region of space. For more information you can visit www.news.nationalgeographic.com/news/2006/08/060824-pluto-planet.html.