well, they don't get notice like they used to, so they are caught pretty much on the hop. DS's school was inspected very recently (and magically achieved 'outstanding' status, too) - and they had one days notice.
BUT once a school is deemed 'good' and has good KS2 results, they get a 'light touch' inspection (that was what happened at DS's school, anyway) and only seem to concentrate on areas suggested for improvment last time. SO , having been told exactly what to do to go up a notch, schools can concentrate on that and get away without too much peering under the beds, I think!
DS's school has a huge proportion of children over the national (and even borough) averages for free school meals, ESOL, and SEN. The level on entry at foundation stage is found to be particularly low, but they do very very well at KS2...mostly it is a happy school - that's what I like!