BtVs by Whedon & M.E., The opening situation is very loosely based on an X-Files episode by Chris Carter. Just using it as a reason to end school early - more a red herring than anything.
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Dawn was sitting in chemistry lab when she heard a weird noise coming up from the drain in the sink next to her.

"Hey, can you hear that?" she asked.

"What? You can hear that from all the way across the room?"

"No. What? Oh, you mean it's over there too?"

Dawn grabbed a pair of tongs and stood up, warily eying the sink just as the first of the rats began to spill upwards into the room.

Chaos ensued along with a lot of high pitched screams as the teacher tried to organize an orderly evacuation before any of the rodents could manage to scrabble out of the sink basins and onto the tables.

Dawn was the third out and the first one to notice the noise of rat feet scratching on the other side of the closed door to the girl's bathroom.

"Great," she announced and pulled the fire alarm.

School was closed for the day and so, after being questioned by a red-haired woman and a black-haired man who claimed to work for the FBI, Dawn caught a ride to her house.

She opened the door and walked in, not seeing anyone.

She moved around the house for a bit until she felt someone staring at her and whirled around to see Willow staring at her.

"What happened?" Dawn asked, concerned, as Willow's blouse was spotted with blood. "Is every-"

"Buffy's been shot. I don't think she's going to make it and I don't have the mojo to help."

"Where is she? Who did it?"

"Warren. She's at the hospital, but I need you to come to the Magic Box with me, so we can figure out what to do. If I get enough energy I might be able to save her."

"Research party, eh?" asked Dawn as she followed Willow out the door.

"Something like that," stated Willow as her eyes flashed black.

...

The bell rang at the Magic Box and Anya turned to greet her newest customers, only to realize with a frown that they were Willow and Dawn.

"I don't have time to talk," Willow said, raising her hand to fling Anya across the room and into unconsciousness.

"Hey," said Dawn as she rushed over to make sure Anya was otherwise okay. "You can't do things like that."

"I needed to," she said in a way that made Dawn study her face carefully.

"What aren't you telling me?"

"Tara's dead. He fired two shots and she was by the window. There was nothing the ambulance could do to help, so there was no point in telling them what had happened. They had enough on their minds already."

"That's horrible. Can I do anything to help?"

"I can use you as a conduit to absorb the energy flowing through these books. It will leave them blank and cost the place some money, but I frankly think it's worth it to heal Buffy and kill the bastard."

"Good," Dawn said and stepped forward to shake Willow's hand. "I'm in. Hey. Too tight," she said, trying to pull away. "Let go. Your nails are digging into my flesh."

"Good," Willow said as a wind whipped through the room. Shoving her free hand into the pile of books on the table, the witch felt her eyes turn pitch black. She raised her head to the ceiling in a scream as she forced the stored energy from the lore books through her body and into Dawn's core.

Magics mixed wildly as the monks' spell that kept the Key in a human form flowed and twisted, disintegrating Dawn into a green mist which Willow happily drank.

Suddenly except for Anya, Willow was alone in the room. She grasped hold of the table as hard as she could, as if that would stop the world from swirling around her.

*POP*

"Oh," said Dawn as she looked down along her re-formed body and realized that, despite the poor fit, she was wearing Willow's clothes. "Crap."