What is the highest average monthly rental of all the students?
SELECT monthly_rental FROM Student_Addresses AS T1 JOIN Students AS T2 ON T1.student_id   =   T2.student_id GROUP BY T1.student_id ORDER BY AVG ( monthly_rental )  DESC LIMIT 1
highest average monthly rental refers to MAX(AVG(monthly_rental)); all the students implies considering every student in the Students table;