Proper lines outside a school (7)
I believe the answer is:
primary
'school' is the definition.
(primary school is a type of school)
'proper lines outside a' is the wordplay.
'proper' becomes 'prim' (prim and proper).
'lines' becomes 'ry' (abbreviation for railway).
'outside' is an insertion indicator (some letters go outside others).
'prim'+'ry'='primry'
'primry' enclosing 'a' is 'PRIMARY'.
(Other definitions for primary that I've seen before include "Of the first order" , "The colours from which all others can be made" , "Paramount" , "Cardinal" , "Principal" .)