Our site school allowed each family to determine its own curriculum; high school families had additional recordkeeping to document state subject matter requirements. Parents had to attend 4 meetings per year, where we turned in attendance records, quarterly evaluations and/or grades if we chose to grade. The school kept the student "cum" files. The cost was around $125 per year to the school.
It was a perfect situation for us. We had complete control over our curriculum. We also had a support group, a HUGE lending library of books and teaching aids, lots of field trip options, etc. We were free to participate in any of these extra activies, or not, as we chose.
oops, forgot to add that we had no state testing requirements in California. I don't know if families who register as their own "private school" have any state testing requirements.