by Richard Oelmann.
That sounds like your teachers have that role site-wide, where ideally, roles such as student and teacher should be given on a per course basis. That would allow them to be teachers wherever they need to be, but students on this course.
You could remove the site wide role and use a csv user upload to add them back to all the individual courses they need as teachers. Then just give them the student role on this one.
Orher than that, possibly a local permission override on that particular activity might be possible?