Three minutes was plenty. Though she had to waste several seconds shooing students away from the doors so she'd have space to bring the next two through, she yanked everyone through with a quick efficiency. Once the last of the students were inside, she phased back out into the hallway.

"Now you guys! Give me your hands!" No need to pull them into the library; she could just make them intangible too, to give them a few minutes to rest without worrying about getting hit by anything.