Prevent what allows youngster to fall asleep in school (6)
I believe the answer is:
scotch
'prevent' is the definition.
(scotching is a kind of preventing)
'what allows youngster to fall asleep in school' is the wordplay.
'what allows youngster to fall asleep' becomes 'cot' (a cot is a baby's bed).
'in' means one lot of letters goes inside another.
'school' becomes 'sch'.
'cot' going into 'sch' is 'SCOTCH'.
(Other definitions for scotch that I've seen before include "providing nursery facility in school" , "A short" , "Cut - wedge - whisky" , "Decisively put an end to" , "Put an end to - in Glasgow? (6)" .)