Originally Posted by Chidori
He's a jonin.
He's the son of Sa***o.
He was trained by Minato.
Why was he hiding during the attack?
He wasn't hiding, the Third Hokage issued that all Ninjas of Kakashi's generation be sequestered away and kept safe during the Kyuubi attack.
Kurenai's father explained it when he said that they were the next in line to inherit the Will of Fire and protect the village, so in the event of a Total Party Kill, there would be people left to continue the Leaf Village, and Kakashi and his classmates just happened to be the perfect age, not too old that they are worth sacrificing against the Kyuubi, and not too young that they wouldn't be optimal replacement leaders of the various village clans.