Former pupils (6)
I believe the answer is:
alumni
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">former pupils</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for alumni that I've seen before include "Former (college) students" , "Schoolkids once" , "Past pupils of the old school" , "Former (school) pupils" , "Graduates of an institution" .)