Short epistle sent up to school (4)
I believe the answer is:
eton
'to school' is the definition.
(famous UK public school)
'short epistle sent up' is the wordplay.
'short epistle' becomes 'note' (a note is a short letter or epistle).
'sent up' shows that the letters should be reversed in order.
'note' written backwards gives 'ETON'.
(Other definitions for eton that I've seen before include "Berkshire town" , "British public school of note" , "old school" , "Town on the Thames noted for its school" , "Posh British public school" .)