Dragon Ball Z, Vol. 20 by Akira Toriyama

Dragon Ball Z fast-forwards into the future! Years after the battle with Cell, the Earth has forgotten about the existence of the super warriors, and Gohan is living the life of a mild- mannered high school student. Mild-mannered until evil strikes,that is...and the world needs the power of the Great Saiyaman!As Gohan's high school classmates ponder the similarity between their classmate and the masked crimefighter, two even stronger warriors prepare to make their names known. Their names are Goten and Trunks, and though they're not even out of elementary school, they may be the toughest things on the planet.