what does scout mean when she says "i thought jem and i would get grown but there wasnt much else left for us except possibly algebra"

I think Scout is talking about the fact that they have lost much of their innocence. The wonder they once held about the world and Boo Radley is gone and they are growing up. The only mysteries left seem to be in things like Algebra.

As much as she's grown up through her experiences, Scout still has a lot of growing up to do. She still has a lot tom learn.