HB 1591 restricts school districts from beginning the school year before the fourth Monday in August, with limited exceptions. The bill eliminates the ability of districts designated as "districts of innovation" to opt out of this requirement through local innovation plans unless their plan is renewed or adopted after the bill's effective date. It allows year-round schools and certain campuses in large districts undergoing comprehensive reform to start earlier. The act applies to the 2026–2027 school year unless a local innovation plan pre-dating the law remains in effect.