It may appear pedantic but "all of them received A minuses or As minus grade", does not appear to make sense.
If a subset of students received the same grade on their reports, then should the grade be pluralized?
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I would rephrase this as "They each received an A-."
Note: I originally said you could use "They all received an A-." Since the reason I tossed that out may be relevant, I'll explain why I changed it. To say "all" instead of "each" could mislead someone into thinking that the students received an A- as a group, rather than each of them individually earning an A-.