1. She is good at dancing.
2. She is good at dance.
(Which one is correct? Are both correct?)
3. I am a student at Class 1-1.
4. I am a student in Class1-1.
5. I am in Class 1-1.
6. I am a first year student in Class
7. I am a first grader of Class 1-1.
(Are all grammatical?)
She is good at dancing. (1)
Yes all are grammatical.
I agree that #1 is correct, but #2 is not. However, in #s 3-7, I agree with everything except #3. In #s 4-7, all phrasing is grammatical, but in #3, the word "at" is not used in this context.