I teach at a very comprehensive school in western Sydney and all the English teachers at my school are marking HSC English this year. None have ever taught at a selective high and two have only ever taught at my school. Similar in Science although not all are marking this year. And I could go on.
In fact the BOSTES makes a point of getting teachers from across the range of school types: comprehensive co-ed, comprehensive single gender, selective co-ed, selective single gender, independent, catholic, rural etc to also maximise the experience of teachers across the spectrum and to ensure that views of a range of teachers can be brought into the equation.