Teacher rearranged the classroom (12)
I believe the answer is:
schoolmaster
'teacher' is the definition.
(I know that teacher can be written as schoolmaster)
'rearranged the classroom' is the wordplay.
'rearranged' is an anagram indicator.
'the'+'classroom'='theclassroom'
'theclassroom' anagrammed gives 'SCHOOLMASTER'.
(Other definitions for schoolmaster that I've seen before include "The classroom (anag)" , "One teaching" , "Male teacher" .)