The Scent of Magic: The Doomspell Trilogy (Book 2)
by Cliff McNish
Rachel and her brother are extraordinary. She is a spell-maker. He is a destroyer of spells. The High Witch, Heebra, wants Rachel and Eric crushed, and her enemies, the Wizard's, destroyed. A party of witches is dispatched to find the thousands of children with powers like Rachel's, and turn them into a ferocious army.
The second instalment of the classic Doomspell Trilogy.