As Petrr suggests, the accessallgroups capability is the cause of this dilemma.
The forum will send notifications to any user who can view a post. If you have the accessallgroups capability then you can view the post and thus you shall receive it.
Either remove the capability from this role at the module, course, category, or site level; or create a new role with the archetype of non editing teacher but this capability removed.
Andrew