1. He is upset because she got a very good grade.
2. He is upset because she got very good grade.
(Do we have to use 'a' before 'very good grade' or not?)
Yes. We use "a" before "very good grade."
The use of "a" before "very good grade" depends on the intended meaning of the sentence.
1. If multiple grades were given, and the person is upset because she received one specific good grade, then the sentence should include "a" before "very good grade": "He is upset because she got a very good grade."
2. If there is only one grade being discussed, and the person is upset that she received a high grade in general, without specifying a particular grade, then "a" is not necessary: "He is upset because she got very good grade."
The use of "a" helps specify that the person is upset about a particular grade, whereas omitting "a" suggests general discontent about receiving a high grade.