Where do they keep all the pillow cases?

Overheard by a friend of mine in the toilets at her office - a work experience girl chatting to a friend:

"We've got this great school trip coming up. But when we get back, everyone's got to stay the last night at school - I don't know why. It's such a pain. I mean, my parents' house can sleep forty, so I might just ask all the girls back to mine instead..."

Forty? What kind of house IS this????