It’s unlikely many of California’s schools will reopen before the summer break, Gov. Newsom said Tuesday, sending shock waves across the state and jarring parents who are just a few days into the widespread closure of classrooms.

The hope has been that schools can open in three weeks, maybe four, after spring break and when the current shelter-in-place policy in six Bay Area counties is scheduled to end. Newsom said the state should start preparing for what would be a worst-case scenario. Closures through the end of the school year would hit disadvantaged and struggling students particularly hard.

“This is a very sober thing to say, and I can’t say this with certainty, but I can say this quite learnedly: Don’t anticipate schools are going to open up in a week. Please don’t anticipate in a few weeks,” he said at an evening press conference. “I don’t want to mislead you. I would plan and assume that it’s unlikely that many of these schools — few, if any — will open before the summer break.”

If Newsom is right, it would mean a five-month break from a formal education for nearly six million students across the state.

Nearly 99% of California students are now out of school, Newsom said, and he expects that a few small districts that remain open will likely soon close. Newsom also said he is seeking a federal waiver that would allow schools to forgo standardized testing for students when they do return. He said exams would be “totally inappropriate.”

The closures have been a local decision across the state, made by school boards and superintendents, and currently that remains in place, said Scott Roark, spokesman for the California Department of Education.

But those decisions are also made in conjunction with health officials and they could recommend a year-ending closure for schools in the future. Such a scenario is a radical departure from less than a week ago when most of the state’s 10,000 schools were still open.

But this crisis has not been predictable and just when politicians or education leaders think they have a plan, new information or new cases have prompted officials to completely reverse course hours later.

Schools will stay open, San Francisco district officials said last Wednesday. And then the next day: All schools will close, those same officials said.

With each update increasing restrictions and closures, the situation has been more dire, so many families say they had already considered the fact that spring and summer breaks could stretch until the first day of school in the fall.

“This could go on for a really long time,” said San Francisco parent Jennifer Butterfoss, who has two kids, ages 4 and 7. “I’ve been urging my fellow mom friends to prepare for the worst.”

In some cases, districts have already started pushing back expected reopening dates.

Riverside County on Tuesday already extended the closure until April 30 after saying last week that schools would close until April 3. West Contra Costa Unified plans to open on April 13, a week later than previously announced.

So far, local district officials say they are focused on getting through the next few weeks, organizing school meal pick-ups and ramping up distance learning opportunities. Most officials said they haven’t formally started thinking beyond that.

And yet: “It is totally dynamic and fluid and we’ve never seen anything like this and we have to be open to all possibilities,” said San Francisco school board President Mark Sanchez. “I think inside of us all we have a deep sense of foreboding.”

San Francisco schools are currently scheduled to reopen on April 3.

The question is whether closing schools and nonessential businesses while requiring residents to shelter in place will work.

The answer at least right now, health experts say, is no one knows.

The current goal is to spread out the demand on the health care system.

“Is three weeks enough? Probably not,” said Dr. George Rutherford, head of the division of infectious disease and epidemiology at UCSF. “You don’t know until you know.”

Everyone will be looking at Wuhan, China, Rutherford said, the site of the first outbreak and where residents are just now coming off shelter-in-place programs and school closures. The first cases were reported there at the end of December.

“We’ll see what happens when they take their foot off the brake,” he said.

In theory, closing schools for two or three weeks would be one full cycle of incubation, meaning those who have it would no longer be infecting others, said Dr. Robert Siegel, a professor of microbiology and immunology at Stanford University.

Yet, there are holes in the system, which may allow the virus to continue circulating in the community — until after schools open again.

But to really know whether schools should open again, the system will need to have much more data on who has COVID-19 — and on who already had it and developed antibodies or immunity to it. But there are no tests yet to identify people with antibodies. Even basic diagnostic testing of those who are actively ill has fallen far short of what’s needed to determine how widely the virus has spread.

Without solid data on how many people have been infected — and whether cases are increasing or decreasing — it will be hard to tell when closing schools and keeping people at home has worked, Siegel said.

“There’s a question of whether or not this will work. I cannot emphasize enough that testing is the key,” the Stanford professor said. “If they haven’t been infected and they go back, but the virus is still percolating in the community, there is a chance we’ve bought ourselves two additional weeks, but it may not turn the tide of the outbreak.”

That delay could be critical to helping officials prepare the health system for an onslaught, but it might not reduce the number of people who eventually get it. The outbreak in California could conceivably stretch into the summer, infectious disease experts say.

“It’s like we’ve hit pause,” Siegel said. “What if all we did was push ourselves further back?”

Researchers are looking at China as well as South Korea and Italy, places where the virus first spiked, for some guidance on what might work to mitigate it.

China implemented massive quarantines while South Korea went with massive testing programs. A combination of those policies might be the best course of action, Siegel said, and that means many hundreds of millions of tests.

“We could make much better policy decisions if we had much better testing,” he said.

School officials say a long-term closure — or one that lasts until fall — would be devastating, especially for students who are the farthest behind academically or have the greatest needs, including special education students and those dealing with stress at home.

“There would be so many repercussions but in particular, with a focus on the role schools play, we’re concerned about learning loss for children whose parents aren’t able to support their continued learning,” said Gentle Blythe, spokeswoman for San Francisco schools.

Education officials want the schools to reopen, but the situation is evolving so fast, it’s hard to say what will happen tomorrow let alone in three weeks.

“There is new information and health guidance all the time,” Blythe said. “We are paying close attention and preparing for various scenarios.”

In San Francisco, Butterfoss has set up routines for her children. There is math time at 11:30 and a mid-morning recess, for example.

While panic set in when she heard schools were shutting down, causing her hands to shake, she realized it was time to buckle in for the duration.

“This is our life now,” she said. “Let’s get homeschooling.”

Yet Butterfoss, a former principal at Alvarado Elementary, said she worries for the students across San Francisco and the country who will only fall further behind because their parents are overwhelmed or working.

“Typically the kids of parents who are able to put in that time and are highly literate themselves, those kids tend to do just fine,” she said. “My heart is absolutely breaking for those communities that are going to be impacted from an educational standpoint. The gaps are already huge.”

Sanchez said superintendents from the six shelter-in-place counties are in communication to determine next steps and calibrate their actions as much as possible.

But he fears that as testing increases, communities will see a big increase in coronavirus cases, which could significantly impact how long kids are out of class.

“I have no reason to believe we won’t see some kind of spike,” he said. “If that were the case, then we are in for a long, horrible, school-less situation for our students.”

Chronicle staff writer Alexei Koseff contributed to this story.

Jill Tucker is a San Francisco Chronicle staff writer. Email: jtucker@sfchronicle.com Twitter: @jilltucker