Thousands of Minnesota students got an extended Martin Luther King Jr. holiday weekend Tuesday as many schools across the state shut down due to extreme cold. Other students were allowed to stay home but had their lessons moved online.

But just how cold does it have to be before in-person school is canceled or switched to an e-learning day?

While there is no state law setting the threshold requiring school districts to close, many "follow the 'law of nature,'" said Jim Skelly, a spokesman with the Anoka-Hennepin School District, the state's largest with about 38,000 students. "That is when temperatures reach minus 35 degrees, that is the time to consider action."

Skelly said the first choice is always to have school, but when the National Weather Service issued a cold weather warning for Anoka County, "that was a key decision point in Anoka-Hennepin's decision," to call off school Tuesday, he said.

Due to the sprawling district's complex transportation system, he added, a two-hour late start was not an option.

An extreme cold warning remained in effect until mid-Tuesday morning for all of Minnesota, where windchill readings were registering between 30 to 50 below zero, the National Weather Service said. Extreme cold warning is the new term for what used to be known as a windchill warning.

"Persons are urged to stay indoors until conditions improve," the Weather Service said.

That was enough for St. Paul Public Schools to shut down for the day on Tuesday. Per district policy, classes and activities will be canceled if windchills are forecast to be minus 35 degrees or colder at 6 a.m. or snow makes it too difficult for students and staff to get to school safely, the district's website says.

It's similar in Robbinsdale, where the district switched to an e-learning day Tuesday since windchill readings at minus 35 degrees met the benchmark to keep its 10,000 students at home,

"It's in that zone that frostbite is possible within 10 to 15 minutes, per that National Weather Service," said spokesman Derrick Williams. "Since most students walk to school or ride a bus, many of them spend 10 to 15 minutes outside in the morning, which puts them at risk during outbreaks of extreme cold like this."

The Rosemount-Apple Valley-Eagan school district will close schools for cold weather if the National Weather Service forecast for 6 a.m. the next day calls for air temperatures of 25 degrees below zero or a windchill of minus 35 or lower, the district's website said.

Though districts set and control when to close or start late, there have been times the state's governor made a unilateral decision. That happened when DFL Gov. Mark Dayton ordered public schools closed on Jan. 6, 2014, as the mercury fell into the minus 20s in the metro with a minus 48-degree windchill, according to the Minnesota Department of Natural Resources.

Republican Gov. Arne Carlson also ordered public schools closed on Jan. 16, 1997, Feb. 2, 1996, and Jan. 18, 1994, because of below-zero temperatures and windchill readings even lower.