I'm in the 9th grade or I'm a 9th grader.

That depends on the country you are in. We don't use 'grade'. I'm in year 9.

It sounds fine to me as a Br Eng speaker, but as you will have seen, Americans prefer 'ninth grade', to 'grade nine'. Either way it is perfectly understandable. FYI in Australia we would also say "I am in year 9" (this refers to the years of primary and secondary school (elementary and high school in American English!).

I am in the ninth grade or I am in ninth grader or I am a freshman in high school