Snatched from Earth! - Bruce Coville

Pleskit's new in school. He's also purple, bald and has a single stalk growing out of the top of his head. It's going to be an interesting first term...

Pleskit, Maktel, Tim and Linnsy are in deep danger: they have accidentally stowed away on an alien spaceship - a ship filled with the Fatherly One's worst enemies. Though they manage to avoid detection on the journey, when they escape, they realise they have left Earth far behind. Stranded on a planet full of enemies, the kids face their toughest challenge yet - how to find their way back home...