I was in the middle of leaving a comment to ask you for the source of your quote because I was questioning the use of disparate as a noun; I've never seen it used that way before. (Merriam-Webster shows it only as an adjective.)
To my surprise, when I looked up disparate at Oxford, I actually discovered your example sentence:
Things so unlike that there is no basis for comparison.
‘The second class of disparates have more to do with the particular foibles of the aperture involved.’
‘Here live disparates, renegades and various isolationists who don't want to join the greater Namqua society.’
The word is listed as archaic, and there is no reference for the origin of the quote.
As for the subject-verb agreement, UK English may refer to collective nouns as either singular or plural. Meanwhile, US English normally only refers to collective nouns as singular. (Aside from the specific case of sports team names that take a plural form.)
It at least seems that this quote originated from a UK source, so the grammar would be acceptable in that country. (But not in the US.)
Having said that, Google Books Ngram Viewer shows that the phrase the class is is more common than the class were. Strangely, even though the British-only corpus shows a more marked usage of the class were, if you switch to the American-only corpus, it shows the class were is more common use than I would have thought.