Well, in this case, you can create groups and give them specific rights.
In the main page, go to
Options > Users > Manage users & groupsHere you can create all the groups that you want.
after this, go to one subforum and click on
options > users > change permissionsYou can edit here the rights for each group(Only for that sub-forum)
You can do this with all subforums and give rights. Notice that this will take you a lot of time, because if you have 90 teachers and you want to give specific rights to each one, you will need to create one group for each teacher!
Regards
:) Helper