Gray Magic (original title: Steel Magic) by Andre Norton, 1st Scholastic paperback printing this title, TX919, 1967, 159 pages. Illustrated by Robin Jacques.
DESCRIPTION:“‘Eric! Greg! Come back!’ cries Sara. But they have already gone through the castle wall, and Sara must follow. Into the strange gray mist. Back into time. Back into the world of King Arthur and the land of Avalon.
Avalon has been captured by the forces of evil.
‘Only you can save us now,’ says the magician Merlin. ‘Only you have the power to set Avalon free!’
Sara and her brothers must not fail- or they will never return to their own century!”(from the back cover)