Her senses were thrown open as she pelted down the stairs and weaved through the corridors—out of Brewers Hall and in through the back of New House, round the shortcut by Modern Foreign Languages and past the locked doors of the arcane labs, out the other side to the colonnade of the main school building. She felt the imp in the staffroom photocopier go, and the haunted radiator in a third-floor Maths classroom, and the peculiar being that made its home in the chapel organ and could sometimes be heard singing windily to itself on moonless nights. One by one they were swallowed up, the canaries in Walden’s personal coal mine. The incursion wards that were all over the school—painted over doors and printed out on posters and carved into the fourteenth-century masonry of the Old Refectory—groaned and began to glow under the strain. A fire alarm started ringing. Walden met a Marshal—not Kenning, one of her squad—standing around uselessly in the colonnade. “Bar the road!” she snarled at him, and kept running.

The tarmac expanse of the main road was a natural barrier, but not much of one. If the incursion jumped across it, there were four hundred children asleep in Scrubs.

Walden stumbled to a halt outside School House. She had already had no doubt. But the sight of the red-brick building swathed in shifting curtains of purplish light confirmed it. An incursion ward was incorporated into the design of the stained glass above the front door. It glowed white, but it was holding.

Gathered in a huddle on the grass outside: Ebele Nwosu, in a dressing gown and bonnet, and four children. Walden slowed as she reached them, counting. There were seven children currently fostered in School House: she could see four. A Year Eight, two Year Nines, a Year Eleven. Ebele grabbed Walden’s arm. “He went back in,” she said.

“Get further back,” said Walden.

“Saffy—”

The door of School House burst off its hinges. The incursion was worse inside, much worse, but the ward over the door hung on; raw magic boiled against the opening and did not get through. Out of the shadows came Ezekiel, right fist closed as he maintained a precisely spherical magician’s shield of pale, silvery light. Over his left shoulder he had the fifth of School House’s seven young sorcerers: a six-year-old boy.

Ebele flew to them. Ezekiel embraced her and handed her the little boy in the same movement. Then he turned to Walden.

“I couldn’t get near the tower,” he said. “Nicola and Mathias both have bedrooms up there.”

Walden looked up at the building. Other people were gathering. The fire alarm had woken everyone up. The tower was a black blotch against the night sky, shrouded in a dull glow of deadly violet. Walden exchanged a look with Ezekiel and saw in his eyes the same grim calculation she was making.