Mark is caught with grass in school (7)
I believe the answer is:
scratch
'mark' is the definition.
(I know that mark can be written as scratch)
'caught with grass in school' is the wordplay.
'caught with' becomes 'c' (cricket abbreviation. I am not sure about the 'with' bit.).
'grass' becomes 'rat' (both can mean a snitch or informer).
'in' indicates putting letters inside.
'school' becomes 'sch' (common abbreviation).
'c'+'rat'='crat'
'crat' placed within 'sch' is 'SCRATCH'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for scratch that I've seen before include "handle superficial irritation" , "Handicap where expected to make" , "Mark with something sharp" , "Withdraw from (competition)" , "Superficial damage; withdraw" .)