Wise men backed international school (4)
I believe the answer is:
magi
'wise men' is the definition.
(the three wise men from the nativity)
'backed international school' is the wordplay.
'backed' shows that the letters should be reversed in order.
'international' becomes 'i' (abbreviation eg in IMF).
'school' becomes 'gam' (both can refer to a group of aquatic mammals).
'i'+'gam'='igam'
'igam' backwards is 'MAGI'.
(Other definitions for magi that I've seen before include "Nativity story wise men" , "Three Wise Men from the East" , "Sorcerers in ancient times" , "Eastern men of lore; Epiphany figures" , "men on special quest" .)