Plan in school making a mark (7)
I believe the answer is:
splotch
'a mark' is the definition.
(thesaurus)
'plan in school' is the wordplay.
'plan' becomes 'plot' (plotting is a kind of planning**).
'in' is an insertion indicator.
'school' becomes 'sch'.
'plot' placed within 'sch' is 'SPLOTCH'.
'making' acts as a link.
(Other definitions for splotch that I've seen before include "Spot, splash, smear" , "Mark" .)