It is common for ninth graders to be 15, just not for them to be 15 on the FIRST day of school. I was 14 when I started and turned 15 in December, so I was 15 for about half of the school year. That's quite common, since most students start ninth grade at age 14, and unless they have a summer birthday, turn 15 during the school year.

If you don't need your character to be 15 on day 1, you could give them an early September birthday, so they'll be 15 for most of the school year.