A local co-op here has about 15 families. The parents teach about 10 courses offered each semester, the kids pick and choose the ones they want to attend. We never had more than 4 families working together. Most of the time, all the moms would stay for the coop, assisting with activities. It was a lot of fun to work and play together.
All children have equal readiness for all subjects at the same time.
One of homeschooling's advantages is tailoring curriculum to our child's temperament, learning style and bent. If homeschoolers always agree to standardized testing for the measurements of our children's success, they we are also agreeing to standardized curriculum. When I had to have my child tested in grade 3 as per our states law requirements, we studied for the test. I tailored my curriculum so he could do well. I sure don't want my curriculum driven by government school standards.