Ive looked in to it, but cannot find a source for this, but i would say it is:
Yes, by changing the ferry, the road is considered closed and will be scored. The player on that road will get the teachers.
Yes, by changing the ferry, a road can be re-opened. The person who closes the road will get the teacher.
The rules do state specificly that, once all 6 roads are completed, the school is connected to the outside world and the teacher has enough students so doesnt go out any more. As mentioned, the school is meant to be played with just base and I&C,so this is something that they didnt plan on
I would say yes, even if all 6 roads have been closed and the teacher is returned to the school. When one of the roads is re-opened and closed, the teacher will go to the player that closed it.
The rules do state that after the roads have been closed the teacher wont leave the school again, so i guess it is a tricky one.
We play with the (house)rule that when re-opened, the teacher can still be claimed.