When Stillman and Jefferies interview Michael and he tells them about the necklace. They bring up Tumi had that same one. However they weren't the ones who interviewed Tumi earlier. So they wouldn't be able to know that.
Gibby refers to Maddy as a freshman at the time of her death. Five years later they interview her classmate Michael at the same high school. Assuming he now a senior this would mean she was in 7th grade when she died.