Should classrooms be open to the world, in the sense that anyone might see what's going on, and perhaps even take part in them? Is this an inevitable development anyway, given today's technology?

These are the sorts of issues that Steve Wheeler raises in his reflections on Open Door Classrooms. 

These kind of questions are usually raised from the standpoint of the potential benefits to students, but it may be worth reflecting on the advantages to teachers too. I always had an open-door policy, and actively encouraged members of my team to walk in and out of each other's lessons, commenting or otherwise contributing as they did so, if they felt so inclined. There were definite advantages to the teacher of this openness:

First, it was always interesting to hear an alternative point of view or explanation.

Second, it lightened the workload a little, if the visitor decided to spend 5 minutes talking to a group of students about their work or giving the class a different example, from their own experience.

Third, it often injected a bit of humour into the proceedings, especially if (as happened to me once)  the visitor tells the class not to listen to such baloney!

Finally, the effect of someone saying something like that is to give students the idea that not everything is cut and dried, that there is not necessarily only one right answer, and that what is arguably even more important than the answers are the questions. How is that an advantage to the teacher? Because a good teacher will be striving to get that point across all the time anyway.

I think the advantages to the students of the suggestions above are obvious.  But Steve is, obviously, talking about an open virtual door; would the same considerations apply? I think they could, as long as the invitation was to participate rather than just observe. Having untold numbers of people  "lurking" but not speaking would be unnerving. If they were anonymous too, it would be unwise. 

An open virtual door classroom is, I believe, a policy to be encouraged. But it would need to be thought through very carefully, and enjoy the support of the leadership team, parents and, of course, the students themselves.