Language is not logic.
A fraction of [collective noun]
used as the subject of a verb is not given the treatment normally accorded to a subject noun modified by an adjectival phrase.
In the normal case, for example
The brother of those two women is also a singer
the subject is singular and determines the number of the verb (also singular) even though the modifying adjectival phrase refers to a non-singular noun (women).
"Fraction of X" is unusual. The number of the noun following after "of" determines the number of the verb. For example
Only one quarter of his students are going on that field trip.
The idea here seems to be that more than one but definitely not all of the students are going, which does not reflect the strict mathematical definition of "fraction." So it is a plural number who are going.
One third of the city is unemployed
does not refer to multiple cities, but to a single city, and so takes a singular verb.
When we get to collective nouns, we can look to the actual number presumably referred to or to the grammatical number of the noun.
One quarter of his class is going on the field trip
is idiomatic as is
One quarter of his class are going on the field trip.
Personally, I prefer the former but I have certainly heard the latter from educated native speakers.