Thanks for your question. Your diagram of the first two sentences is correct:
Unpopular with faculty Modify policy
Unpopular with students Adopt a new policy
The third sentence is key: the policy is bound to be unpopular either
with the faculty or
with the students. Logically speaking, an "either...or" construct does not preclude the possibility of both things happening, and so it is possible that the parking policy is unpopular with both students and faculty. However, it must be unpopular with at least one
of these two constituencies. So, if the policy is not
unpopular (i.e. if it's popular) with students, then it must be unpopular with faculty, and vice versa:
Popular with students Unpopular with faculty
Popular with faculty Unpopular with students
Either A or B
is equivalent in meaning to:
NOT A B
NOT B A
The rule does not prohibit the possibility of both A and B occurring (for that to be the case, test-makers must specify "either A or B, but not both"), but it forces at least one of them to occur.
Does that make sense? Let me know.