I'm in 6th grade and want to skip the seventh. I already know all of the material and I'm not being challenged.

Have you take a test to pass the 7th grade? (How do you know everything in that grade already?)

Each school district may have it owns requirements, some will have a specific test, in some you will take the national test your state uses, like the CAT, or the CRCT or others.
They will look at the score on the national tests for your own grade first, and your scores for your current course work.