Group of schoolchildren finding brook (6)
I believe the answer is:
stream
I believe this is a double definition.
'group of schoolchildren' is the first definition.
(I've seen this before)
'brook' is the second definition.
(I've seen this in another clue)
'finding' acts as a link.
(Other definitions for stream that I've seen before include "Small narrow river" , "Natural body of running water" , "not taken in mixed-ability group" , "flower" , "One small river" .)