"I am in eight grade."
Translation:Eu estou na oitava série.
Of course, English speakers in many parts of the world work with different educational systems, and it could be that there is some place where "I am in eight grade " would not mark you out as a person speaking English as a foreign language; BUT "I am in the eighth grade" or "I am in Grade 8" is how it would come out in most of the countries I know about.
"I'm in grade eight."
That's how we say it in Canada. "I'm in the eighth grade" sounds pretty stiff and formal to me. If I was still in grade eight, I would avoid any kid who talked like that.