K9 would have people wondering if it's the ninth year of kindergarten, lol. We say 9th grade, and the kids are freshmen in high school. All grades in the US are given as ordinal numbers after kindergarten, starting with 1st grade.
It might be regional but where I live I never hear "grade #" it's always #th grade both spoken and written.
I was called out as a Canadian one time for saying I teach grade four. I had never noticed that it was more common for Americans to say fourth grade.
K8 generally means qualified to teach K through 8...my certification is k-8, but I'm not highly qualified beyond grade 5. K9 means nothing in the US except for an abbreviation for canine.