1 . I'm in 9th grade 2. I'm a 9th grade student

#4 and #6 are wrong. The other ones are all good. No difference in meaning between #1 and #2. In 9th grade and in the 9th grade are the same thing. It makes more sense to drop the word "the" unless you are going to tell more about your school in following sentences.

1,2,3 and 5