Everyone attending is of normal intelligence (3,5)
I believe the answer is:
all there
I believe this is a double definition.
'everyone attending' is the first definition.
(I've seen this before)
'of normal intelligence' is the second definition.
(as opposed to 'not all there')
'is' is the link.
(Other definitions for all there that I've seen before include "sensible" , "Completely sane" , "on one's trolley, presumably?" , "Having one's wits about one (informal)" , "of normal intelligence" .)