Chills diamonds at school (4)
I believe the answer is:
ices
'chills' is the definition.
(icing is a kind of chilling)
'diamonds at school' is the wordplay.
'diamonds' becomes 'ice' (slang term for diamonds).
'at' means one lot of letters go next to another.
'school' becomes 'S'.
'ice'+'s'='ICES'
(Other definitions for ices that I've seen before include "Freezes" , "Frozen desserts" , "ecorates with frosting" , "Frozen sweets" , "Decorates cake with sugar topping" .)