It's enshrined somewhere on the 10 month contract amount with the state. This is from TCTA's website addressing this:

"Required days of teacher service

The 10-month teacher contract year requires a minimum of 187 days of service. Recent TCTA-initiated legislation allows school districts anticipating providing fewer than 180 instructional days during a school year to also reduce the number of required teacher days of service proportionately without a reduction in an educator’s salary."