The Sixth Kingdom - Wyvern Master Chronicles, #1

He who controls the Wyverns, rules the world. The Sixth Kingdom is the center of the known world.  It is the repository of the all knowledge and secrets, and the heart of arcane wizardry.  It is the College, the arbiter and enforcer of world peace.  It's also the place to learn magic, perfect deadly warrior skills, or bond with dragons. Comprised of four Castes, the College accepts only the best and brightest of those who can afford to send their children to the hallowed halls of destiny where they are groomed to assume their future roles in each of the five kingdoms.
Four friends in four Castes. they call themselves 'the Family.'  Their goal?  To one day rule the College. Of the four, Fiach has the least claim to privilege as he was sold to the College by slave traders after his parents were murdered.  It is a memory he can no longer repress as he and the Family are swept up into the power struggles to rule the world.