If we take the following table, ignoring other irrelevant attributes:
I think the rows returned will be 17,18 and 20 because for them, there exists no student who is less than 18 and has a higher rank than them. (Eg: The student with age 17 will be compared row wise with students with age 15,16 and 17. Now since the ranks of 17 is not less than any of them, none of the values will be selected, which will make NOT EXISTS to be true.)
So I think it should be None?
If you see any mistake, do point out.