"I guess we could wait until the school empties. Or - if it's been hanging out by the boiler then it probably needs water? Or warmth?"

"Ah huh. If we send something cold through the vents maybe it'll come to us."

"Radio-controlled car and... ice, I guess? Although you'd need a camera on it to have any idea which way it should be going. Or a map of the vent system, maybe, but I have no idea where we get one of those. Sort of wildly spitballing here."

"You're thinking about the right stuff, though. Do you know where the air intake for the ventilation system is?"

"Nah. One of the janitors might, I guess? Someone has to upkeep the system, I'd think..."

"Yeah. Let's go ask them."

The janitor they find needs to check his binder - Sunnydale janitors change so frequently that apparently Principal Flutie has taken to giving them all binders of everything they could possibly be expected to know, rather than wait for them to actually learn all of it. He then tells them where the air intake is, and doesn't even bother to ask them what they need to know for.

That's convenient. "There's liquid nitrogen in the car. I think the easiest thing is to make the whole school very chilly for a little bit, and wait for our slime thing to come for the boiler."

"That makes sense. You have a lot of things in your car."

"Oh, the antiaircraft missiles aren't in the car, I have to call in for those." He stacks several identical indistiguishable black boxes and pulls out some metal canisters. "Here we go."

"Solid. I don't know how to handle those safely so I'm going to let you do that and then wait by the boiler, I think."

"Sounds good." He opens the canisters outside the air intake valves; they start to steam immediately. 

The school is pretty cold by the time the Karen makes it to the boiler. The demon, it turns out, does not like cold. After a couple minutes of waiting, there's a gurgling noise, and then the vent cover is punched clean off the wall, and then the demon with lots of claws is headed right for Karen.

Between Karen and the claws and the baseball bat, the baseball bat is the weakest link; it splinters.

She takes - really almost no time to process this, actually, before skewering the thing's center on the pointy piece of wood she now has in her hand. The demon shrieks.

He has arrived at the door by then but hangs back watching.

She takes her stake out and stabs it again, on the theory that this will probably do more damage than leaving it in.

The demon explodes. Its innards are horrifically messy, but apparently not immediately caustic, because Karen is covered in them and mostly seems grossed out.

"I.... think it's dead."

"Yes, I think so. Well done."

"Thank you. Uh. Do we do anything about the dead part-of-a-body, or...?"

"Taking a shower might be more urgent, but yeah, we should probably not leave this for the janitors. I'll mop it up."

"Yeah. I'll go... do that. Thanks."

She heads for the locker rooms.

She runs into someone on the way.

"...I'm gonna let you slide this one time, Teller."

"That's big of you."

She showers. She changes into her gym clothes, because these are the only clean clothes she has right now. It occurs to her that they are probably in danger of being late for dinner, so she stuffs her slimed clothes into a plastic bag and grabs her backpack and runs out to the car.

If he disposed of the demon he somehow did it without sliming himself at all. He's packing things back into the spacious trunk of the car. "Hey! Are we late for dinner?"

She glances at her watch (which has been carefully washed free of slime, at least on the outside). "We will be, a little, by the time we drive over there. It's not a huge deal."

"Okey dokey." He gets in the car. "You did great."

"Thanks," she sighs, without much enthusiasm. "I guess we need a new gym teacher now."