You wrote:
Right now, the tests only judge proficiency at grade level. My older child probably would have passed the fifth grade proficiency exam years ago, but instead, each year all he got to show was that he was proficient at first grade skills, proficient at 2nd grade skills, etc. As long as he showed as proficient, no one really cared that he'd learned next to nothing all year in class and that almost all his learning was done outside of school.

I know what you mean; I brought that very thought up to our school's staff. I had noticed that most of my family could be any number of professionals; however they always lack credentials. I want my children to get the notice and credentials, for their abilities, that they deserve. The only way that will happen is if I find a school willing to let them go at their own pace; which means individualized instruction.