Thanks for all the responses and your patience. The least that I can say after many comments in this forum thread is that separate groups with email notifications enabled don’t seem to be consistent within the Moodle activities. The teacher in the same course will receive emails from separate groups in a Forum activity (where he or she is not a member of any group), but not in Feedback activity. This is what I observed and tested in the last days in Moodle 2.2 and 2.4.
The good news is that I found a solution. As the teacher is able to receive emails in the Feedback activity when the group mode is set to visible, but the student will only see his or her answers, but not the answers of other groups, I will go with this approach. Thanks again, Andreas, for providing this module and for the forum moderation.
With kind regards,