Sean Johnson, the lobbyist for the state's teachers union, said it "codified the brain drain" that occurs during summer vacation.
But Hogan and Democratic Comptroller Peter Franchot, a key ally, said the longer summer recess would give families more time together, generate more revenue for the tourism industry and help keep students in the Baltimore region out of sweltering classrooms that lack air conditioning.
The executive order overrides the traditional local control over the school calendar. It is likely to inspire a political fight that will stretch into next year's legislative session.
The debate has pitted tourism and business interests eager for more family travel against education leaders and politicians who say shifting the school year hurts students and teachers.
Baltimore County schools Superintendent Dallas Dance said the order runs counter to prevailing wisdom in education circles.
"Many of us are trying to think about 'How do we give our kids more time in school?'" Dance said. He said his district might have to eliminate time off for religious holidays and trim spring break to meet the terms of the order.
Hogan and Franchot dismissed claims that the order was illegal.
"The [Maryland] Constitution clearly gives us the right to do this," Hogan said. "There is no legal argument whatsoever."
He said he received advice from his legal counsel, but declined to release it.
Franchot said it was Hogan's "responsibility to have a great school system and a great business community.
Franchot launched a "Let Summer Be Summer" initiative about two years ago, and Hogan signed onto the idea shortly after he was elected in 2014.
Pollsters at Goucher College last year found that 72 percent of Maryland residents supported a statewide mandate to require schools to begin after Labor Day. Supporters said it would be better for families and students, that starting earlier didn't make sense, and that the school calendar should return to the way it used to be.
Traditionally, school districts began school after the first Monday in September. In many districts, the start date began to creep into August during the 1990s, as school officials grappled with a state-mandated 180-day instructional year.
State law still requires a minimum of 180 school days each year. Hogan's order says districts can get a waiver that allows some of those days to fall before Labor Day or after June 15, so long as they provide "compelling justification."
Andy Smarick, president of the Maryland State Board of Education, which would grant the waivers, said it was the board's job "to make this work for schools."
"This is not rocket science," Franchot said. "They have to start after Labor Day and get out by the middle of June. In between, they can do anything they want with their schedules."
Ocean City Mayor Rick Meehan said Worcester County accommodated the switch to a post-Labor Day start "seamlessly" three years ago.
"Ocean City is part of the reason behind this, but it's a positive move for the whole state," Meehan said. "Instead of people putting together reasons they can't do this, if they start thinking about ways they can do this, they'll see this is something we can all accomplish."
The new start and stop dates left school officials elsewhere in Maryland pondering how to accommodate religious holidays and teacher development days while properly preparing students for standardized tests and figuring out how to help impoverished communities that depend on schools to feed children.
"It's going to be a real challenge," said Alison Perkins-Cohen, chief of staff of the Baltimore school system.
Eighty-four percent of public school students in Baltimore qualify for subsidized meals, and many rely on the school system for at least two meals a day. Leaders in the district, one of the lowest-performing in the state, already are concerned about learning loss during the summer — a problem they say is compounded by a dearth of summer jobs or other constructive activities.
"A lot of our students don't have access to vacations in Ocean City," Perkins-Cohen said.
State Sen. Paul G. Pinsky, vice chairman of the Education Health and Environmental Affairs Committee, said he found the order "unbelievable, actually."
"What that means is what had been the minimum of 180 days of education is going to become the maximum," said the Prince George's County Democrat, a former educator. "There's a bit of a patronizing attitude that the business people of Ocean City can tell the people of Baltimore City how to run the schools. …
"For those people trying to add more rigor to the academic calendar, it sends a pretty bad message."
The section of the Maryland Constitution that grants the governor authority to change law by executive order also allows the General Assembly to reverse some of those actions. It was unclear Wednesday how lawmakers would respond to the order, but they promised to do so.
State Sen. Joan Carter Conway, the Baltimore Democrat who chairs a committee that repeatedly killed legislation that would have required schools to open after Labor Day, predicted the Democratic-controlled legislature would return control of the school calendar to local districts.
Hogan said legislators who reversed his order "would probably lose their jobs."
Sen. James N. Mathias Jr., a Democrat from Ocean City who has spent years trying to persuade colleagues to keep schools closed until after Labor Day, called Hogan's order "a dream come true."
Matthew Boulay, founder and CEO of National Summer Learning Association, said the new schedule would hurt the state's most vulnerable students.
"We have this myth about what summer vacation is," he said. "It goes back to Huck Finn and Mark Twain."
The reality is different, he said.
"Many children look forward to the school year, and it is a stable, loving environment, and it makes no sense to delay it."
Karl Alexander, a sociologist at the Johns Hopkins University, followed students in Baltimore over two decades. His research showed they lose an average of two months of learning over the summer. The loss is greatest for children from low-income families who don't have access to summer camps and other enriching activities.
This year, only one school district in Maryland — Worcester County, home to Ocean City — chose to start after Labor Day.
For Baltimore County mother Nicki Bollhorst, starting school after Labor Day didn't seem like a difficult problem to solve.
"We did it when we were in high school," she said. "I think they can figure it out."
She and her family spend summers in Ocean City. Hogan gave her 10-year-old son, Ben, a note excusing him from school Wednesday so he could attend the news conference.
"We like the extra 10 days at the beach," she said.
Baltimore Sun reporter Erica L. Green contributed to this article.