“Parents who believe their children aren't ready for kindergarten would be able to opt out.”
Home schooling in Michigan...
Michigan parents have the right to home school their children. The law requires a parent or legal guardian of a child from the age of six to sixteen to send his or her child to school during the entire school year, except under certain limited circumstance's. Michigan parents may choose to home school under the home school statute or as a nonpublic school or both.
Granted, GOOD private school kindergartens are beneficial for most; however, those who would send their their progeny to such schools WILL do that sans a mandate.
For the rest? It's a free babysitting arrangement for everyone else.