Group of swimmers in educational establishment (6)
I believe the answer is:
school
I believe this clue is a double definition.
'group of swimmers' is the first definition.
(school can mean a group of fish)
'educational establishment' is the second definition.
(I've seen this before)
'in' acts as a link.
(Other definitions for school that I've seen before include "Where one teaches" , "Train (of thought?)" , "Discipline" , "Train - group of porpoises" , "Centre of learning" .)