Senior pupil (7)
(Other definitions for monitor that I've seen before include "Police" , "Watch - lizard - warship - VDU" , "Supervisor - lizard" , "Keep under surveillance" , "School pupil assigned responsibilities" .)
(Other definitions for prefect that I've seen before include "Schoolchild given authority over others" , "Pupil with authority" , "School senior" , "French official and senior pupil" , "Chief admin officer of a district in some countries" .)
(Other definitions for sixth-former that I've seen before include "Senior school student" , "A-level student?" , "Pupil" .)