(NEXSTAR) – It may be the dog days of summer, but it's already time for millions of children around the country to head back to class.

The once-common practice of going back to school after Labor Day has grown rare in 2025. In fact, some schools have been back in session for weeks.

Schools in Arizona welcomed students back as early as July 16 this year. In Jackson, Mississippi, kids went back on July 28. Elementary schoolers in Chula Vista, California, were in their desks starting July 23.

A Pew Research study of 1,500 public school districts in 2023 found wide regional variation in when kids head back to school. The earliest districts resumed instruction in mid-July and the latest start dates were in early September. The most common first week of school that year was Aug. 14 to 18.

Schools in the South and Southwest tend to start earlier, Pew found, while New England and Northeast U.S. districts resume the latest. However, there are big exceptions within states and even differences between neighboring districts.

Over time the school year has been creeping earlier and earlier. In the 1980s it was more common to start after Labor Day, but August start dates became more common in the 1990s, according to CNN.

Why the earlier start?

Some schools have adjusted the start of the school year as they add more days off later in the year. In Oakland, California, for example, public schools now have a full week off for Thanksgiving, the San Francisco Chronicle reports. It moved the first day of school up to offset the added vacation.

A local principal, Jonathan Mayer, also told the Chronicle that an August back-to-school date makes some logistics easier. An earlier start means more time to prepare for standardized testing that takes place in the spring, he said.

Some districts in hotter climates may also be motivated to get kids back into classrooms, where they know they'll have air conditioning, during the hottest months of the year, Arizona Agenda reports.

Finally, an August start date typically aligns better with academic calendars of colleges and universities, which creates fewer conflicts for families with younger and older kids.

Some states have bucked the trend of earlier and earlier first days of school by making them illegal. Minnesota, for example, requires schools to start after the Labor Day holiday in September. Supporters of the law say the longer summer helps boost the state's tourism industry.

Wisconsin also requires public schools to start after Sept. 1, but exceptions can be granted. Maryland used to have a law forcing a post-Labor Day start, but it was overturned.