Ends school period in one (7)
I believe the answer is:
termini
'ends' is the definition.
(terminus is a kind of end)
'school period in one' is the wordplay.
'school period' becomes 'term' (I've seen this before).
'one' becomes 'i' (Roman numeral).
'term'+'in'+'i'='TERMINI'
(Other definitions for termini that I've seen before include "End stations" , "Ends, extremities" , "End points of rail or bus routes" , "limits of service" , "End-points" .)