Pluto is the ninth planet

What actually happened

Pluto did not shrink, move, or do anything wrong. The word did the changing. Astronomers had never formally defined "planet" until the discovery of Eris in 2005 forced the question: Eris was roughly Pluto's size and further out, so either it was a tenth planet, or Pluto was something else.

The IAU settled on three criteria. A planet orbits the Sun, has enough gravity to pull itself into a round shape, and has "cleared the neighborhood" around its orbit. Pluto satisfies the first two. It fails the third: it shares its region with countless other Kuiper Belt objects and is not gravitationally dominant there. Bodies that meet the first two tests but not the third became a new category, dwarf planets, which also includes Eris, Ceres, Makemake, and Haumea.

This is the cleanest example on this site of a reclassification rather than a disproof. Nothing your teacher told you about Pluto's size, orbit, or composition was wrong. The filing cabinet was reorganized.