"Teachers will receive a message (of a type they choose) whenever a student submits an assignment. Please note if you are using the assignment in Groups Mode (eg Separate or Visible Groups), the teacher will need to be a member of the group in order to receive the submission notification."
Is the part about Visible Groups really true? Didn't work for me. Though in Separate Groups mode the notifications arrived only to the teachers in the same group with the student.