If you mean publishing/submission guidelines, directed to a publisher, the standard would still be the same - Courier, 1" margins, etc. - or whatever the publisher/agent prefers.
If you mean by vocabulary and style, which Anders has already covered, think about 9th grade as high school. High school students are reading literature classics for credit - Twain, Dickens, Nabokov, Goldberg, Homer - which normally considered adult reading levels.
Even when I was younger (junior high, even upper elementary to an extent) and I was reading, it bothered me when I could tell the language was obviously altered to match my age. The characters would speak just like the age group - "Gosh, like, did you see that?" - which drove me nuts.
On average, like Anders already mentioned, high school students who read for pleasure are probably more interested in adult themes and looking for something a bit more challenging, so I wouldn't worry about the age group.
Good luck with your first impression!