>Many of my colleagues do prep work in unoccupied classrooms or work at home or do prep at odd times when the staff room is quiet. This, of course, short circuits the interaction argument for open plan offices.

yep, looking at the number of empty desks in our newly converted (with big pompousness touting increased collaboration and innovation) offices i see the same effect. Personally, i also decreased my office hours and when in the office i usually with headphones on enjoying 80ies metal