In the Southeast Missourian (via Combest) an article discusses a new state law that would require school districts not to start earlier than 10 days before Labor Day:

School districts will have to do more work if they want to start school in mid-August next year. At a ceremony in Branson on Monday, Lt. Gov. Peter Kinder signed a bill
requiring schools to begin their school year no sooner than 10 days
before Labor Day.

The caveat to this new law is that school districts who want to start earlier would have to have a public meeting and voter approval every year to set the start date of schools before the 10-day requirement. Again, the state is taking local decisionmaking outside of the school districts, which would know better than the state of Missouri when to start school to make sure their students receive the best educations.