Will inspectors observe all teachers in a school?

This will depend on the nature and size of the school inspected.  In a small primary school, it is likely that inspectors will observe all the teachers.  In a large primary school and in secondary schools, the inspection team will sample the work of teachers.  Observations may take a variety of forms, e.g. as part of a ‘learning walk’ by inspectors or as a formal lesson observation over a more extended period of time.