I missed the beginning. I ask because her female classmates had very developed bodies, what with their perky breast and bulbous buttocks and all. Meanwhile, young Sadie had the modest and unassuming body of a child.
I'm a real kewl kat.🐈
At the time the movie was released Emma Hentschel, the actor who played the part of Sadie was 13 years old, so she was definitely in the Middle School age range. She does look younger than some of the other girls in her movie classroom but that's not unusual in middle school.
Also, the fact that she looks younger makes her a more sympathetic character when the other kids start picking on her. As for "skipping grades"--since she she was home schooled by her psycho grandmother she would have probably been placed in a class appropriate for her age, not her academic level. Since she needed a math tutor I assume that Sadie would not have been a candidate for "skipping" though she sure did show a lot of intelligence and ingenuity outside of the classroom. :-)
--Ex Teacher