login as admin. turn on Edit. click on edit forum.
click on "Locally assigned roles Tab".
from here you should see all the groups you have. click on the group and you will be able to add the teacher who is in charge of that group for that forum to be emailed.
for students to get an email i believe the setting is under users->permissions->define roles->student.
check if all the settings are correct here.
hope this helps.