Slambash wangs of a comp gormer by Robert Leeson 1987 hardcover

To his schoolmates, Arnold Radleight is Wally of the Year. But in his dreams it's very different: he is Replic Dornal of Klaptonia, prince and heroic victor of countless mortal combats.

What Arnold doesn't know is that Klaptonia really exists. So does Replic Dornal, and he's in trouble. Somehow he must find a safe place to hide, but without anyone knowing he's gone... Which is how Arnold finds himself running for dear life across the deadly wastes of the Bashi Rangles, while Dornal discovers that the Third Year at Denfield is tougher than he expected.