From the SAT:
To those of us who had heard the principal of the high school talk about the budget, the news of the staff cuts was not surprising.
Why is it had and not have? Because isn't those plural?
Had heard is cast in the Past Perfect tense, to make it "deeper in the past" compared with the Simple Past construction "was not surprising". There is no plural-singular distinction for the had verb in Past Perfect.
"To those of us who have heard the principal the news seems grim", said John.
From the standpoint of John, the time when he spoke was present. This is why John used the Present Perfect construction have heard, and since those is plural, he used have.
John said that to those of them who had heard the principal the news seemed grim.
Now we are reporting what John said. We backshift seem to seemed (Simple Past), and backshift have heard even deeper in the past, making it had heard (Past Perfect).
You seem to be confused about the applicability of the past tense verb form "had" (and, by extension, the past perfect "had heard") to plural subjects. While the present tense form "have" does have a separate form "has" for third-person singular subjects, no such separate form exists for "had":
- I have heard.
- You have heard.
- He/she has heard. (← distinct third person singular form)
- We have heard.
- You have heard.
- They have heard.
- I had heard.
- You had heard.
- He/she had heard.
- We had heard.
- You had heard.
- They had heard.
This is actually a general feature of English verb morphology, not just something peculiar to the verb "have". Compare e.g. "I walk", "he/she walks", "they walk" in the present tense, but "I walked", "he/she walked", "they walked" in the past tense.
Ps. In your question, you've also emphasized the singular third-person verb "was", whose subject in the sentence is "the news". The reason for that apparent concordance failure is that, despite looking like a plural (and, historically, originally being one), the word "news" in modern English is uncountable* (just like "water" or "information" or "happiness") and thus takes singular verb forms. So:
- OK: The news is good.
The news are good.("News" is not plural.)
- OK: I've heard news from home.
- OK: I've heard some news from home.
I've heard a news from home.("News" takes no indefinite article.)
*) The Wiktionary page I linked to above does list the rare plural form "newses" for "news". While it's not really surprising that such a plural should exist (as pretty much all "uncountable" English nouns can be pluralized in special situations, e.g. when referring to multiple distinct collections of the uncountable substance), in my personal experience I must say that it is very rare, and not something you should normally use. Even when speaking of multiple distinct items or sets of news (as in "today's and yesterday's news"), the word "news" still normally remains uncountable, and still takes singular verb forms.
We may reasonably infer that the context here establishes a'Reference Time' —the time that is being talked about—as the time when the cuts were announced.
At that time, the cuts were surprising to most people, but not to those people who had heard the principal speak about the budget at some earlier time.
Had heard is a past perfect: the event it names is one which occurred before Reference Time and gives rise to a state at Reference Time: in this case, a state of 'non-surprise'.
Have does not inflect for number in its past-tense form; had is used with all numbers and persons.