Sword of the Rightful King — Jane Yolen

THE MAN WHO WOULD BE KING

NEWLY CROWNED King Arthur rules the kingdom ... but not the people's hearts. Unless he proves his worth, his power will always be in question. Too many want him dead, and treachery is everywhere.

So the wizard Merlinnus secretly creates a test for his Arthur....

A sword in a stone is discovered--a stone that bears a legend promising that whosoever draws out the sword will rule England. If all goes as planned, King Arthur will draw the blade from the stone (with the help of magic from Merlinnus, of course), and the people will at last rally around the young king.

Except someone else pulls the sword out first....