Derek had been out checking on the creatures when the Mark had gone up, and it had taken everything in him not to throw up. He'd cast a protection spell over the creatures and then ran into the school to herd the students up.
In the middle of the chaos in the entrance hall, he heard barking and looked up, seeing a small white and gray creature huddled in a corner. "What the fuck," he said, running over and scooping Batman up. "How did you get out? Whatever, come on, hold still." He tucked Batman in against his side and continued trying to get the panicking students to safety.
Once the bulk of the students were locked away, he set Batman down. "I'm going to go check the halls for stray students - knowing your owner he didn't listen to the Headmaster and is hiding out somewhere. If we find him, you bite him, okay?" He nodded and then tossed his head to tell Batman to follow him, and together they began to sweep the halls.
Luckily for Stiles, they didn't find him in the hallways, and after the teachers agreed on a schedule for patrolling the halls and grounds, he set off for the Great Hall, figuring he'd start there and work his way down to the Slytherin common room.