Dozens of districts due to start classes this week have moved their starting date to Tuesday, particularly in hard-hit shoreline and eastern Connecticut communities.
Superintendents and other officials say it will force the districts to adjust schedules to absorb this week's lost school days, possibly by shortening vacations.
State law requires districts to have at least 180 days of classes and dismiss students by June 30.
Officials say they're hoping for a mild winter so they don't face the time crunch caused by last winter's many snow days.
Several districts also are rearranging sports schedules as they wait for standing water to recede from their athletics fields.
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Parents have had to scramble to find child care for kids who were supposed to be in school but now will be hanging around the house longer than expected.
"I hired baby sitters for the summer, but they're done now," said Tara Coleran, of Whitman, Mass., who has been busy searching for someone to watch her three boys this week because their first day of school, originally scheduled for Wednesday, has been delayed until Tuesday, the day after Labor Day.
Coleran, a bookkeeper for a nonprofit, said she expects to miss up to three days of work because she can't find a babysitter.
"I know people in the area who I could ask, but everybody has no power, so it's difficult," she said.
The school year is also expected to start late in other districts including some in Connecticut, Delaware, Maryland, Rhode Island and Vermont.
An extra day or week of summer vacation may be fun for kids, but the calendar reshuffling has caused problems for school administrators who must now reset schedules so students can make up the missed days either during the school year or at the end.
School officials in the Massachusetts communities of Whitman, Hanson, Marlborough, East Bridgewater and Springfield were among those who decided to put off the start of school for one to three days because schools, homes or both were still without power Tuesday. School officials also said they were not comfortable opening and allowing children to walk to schools while utility crews are still removing downed power lines.
"In a nutshell, it's just the lack of power and making sure we keep everybody safe and have everything ready for students when they come back," said Marlborough Superintendent Anthony Pope, who decided to push back the start date for the city's 4,700 students from Wednesday until next Tuesday.
"I have three kids of my own and I know how it is getting kids ready for the beginning of the school year, so we want to make sure that parents have that opportunity to get their children off to a good start," he said.
The school year has been postponed in some districts in Vermont, a landlocked state that was perhaps the hardest hit by Irene, then a tropical storm, with many roads washed out and entire communities cut off from the outside world.
At least five Vermont schools were closed until further notice and about 120 delayed opening for the school year because of roads or buildings ravaged by flooding.
Vermont Department of Education spokeswoman Jill Remick said some schools have been damaged and other communities have such significant road damage that school administrators are considering long-term alternate methods of transportation or instruction.
One Vermont elementary school - The Moretown School - was serving as a shelter on Sunday for people pushed out of their homes by the storm, but it had to be evacuated after the building's septic system backed up, covering the floors with raw sewage.
Principal Duane Pierson said all the carpets would have to be replaced and he doesn't know when classes will start.
On Wednesday, teachers from the school were out helping homeowners in the village about 15 miles southwest of Montpelier clean out their houses.
"It's about being present and doing all we can to help," he said.
In Rhode Island, more than a dozen public school districts have put off the first day of school. More than 30 daycare centers and pre-schools remained closed Tuesday because of power outages.
"We've had no damage to the schools but there are just too many unsafe conditions out there," said Robert McIntyre, superintendent of schools in Barrington, R.I. "I just want to give the cleanup crews more time to get out there and I didn't want to put buses on the road."
McIntyre said three schools were still without power Tuesday.
School had been scheduled to begin Monday, but the start has been delayed more than a week, until Tuesday, the day after Labor Day.
Jenna Young, 7, of Kingstown, R.I., was supposed to start second grade on Tuesday. Now, she's not going back to school until next week.
She and her family had a picnic and enjoyed an extra day of summer at Viscoli Park in Providence on Tuesday.
"If they say it's fit for the kids to be there, you just have to trust them," said Young's father, De Kim.
In Maryland, friends Brandy Mosby, 26, and Heather Comer, 25, hopped on the subway and headed to Baltimore's Inner Harbor on Tuesday on the second day of canceled schools for their children.
The moms and four children grabbed a dinner and then did some sightseeing along the waterfront.
Mosby, who stays at home, has two daughters, ages 10 months and 9 years old. Comer, a hairdresser, has two boys, ages 5 and 9.
Mosby thought the schools should have to get generators to make sure students weren't kept out of school in situations like this.
Two school districts in southern Maryland were the only ones in the state that would still be closed on Wednesday due to flooding.
In Connecticut, 42 of the state's 166 school districts had been slated to start the new year Monday, followed by dozens more on Tuesday.
Mark Linabury, a spokesman for the state Department of Education, said school officials were still tallying the number of districts that delayed their start dates to Wednesday and later. He said the school districts were grappling with power outages, flooding, road closures and disruption to bus routes - all caused by the remnants of Hurricane Irene.
"It's such an uncertain situation right now in many districts based on all of the circumstances," said Linabury.
Parents of younger children also were affected by the aftermath of the storm as daycare centers were forced to close because they had no electricity.
Megan Karavish, 36, of South Glastonbury, Conn., said she had to use a vacation day Monday to care for her 2-year-old son and 1-year-old daughter because their daycare was closed. She expected to shell out more than $100 for the babysitter she felt lucky enough to find Tuesday.
"The whole situation is very stressful, and quite frankly the extra costs this is bringing on is really adding to the stress," said Karavish, a tax software sales representative.
"The fact that there's no answer about how many more days it'll last is really what's the worst part. There's just no way to plan ahead."
Colleges were not spared, either.
The storm delayed student move-ins and early classes on a number of campuses, though by Tuesday most appeared to be up and running.
Two State University of New York campuses were still without power Tuesday, spokesman Morgan Hook said. Several were dealing with flooding.
The University of Vermont escaped serious damage, though some colleges farther south in the state were partly cut off by road flooding. Water covered playing fields and poured into an athletics building at Castleton State, rising to 56 inches in the football team's locker room and leaving pads strewn about the mud when the water receded.
One big challenge was that flooding at a state office building left public college campuses around Vermont with only intermittent Internet access. Students at numerous schools struggled to reach campus, while the schools offered assurances professors would understand if they missed early classes.
Marlboro College - a small private college in south-central Vermont - was still evaluating whether to start classes Thursday, and changed to a rolling course registration process to accommodate students who couldn't arrive in time.
Many school districts with planned openings after Labor Day are expecting to open on time, even though some are still struggling with power outages.
In Richmond, Va., six schools were still without electricity Tuesday, but since classes don't start until next week, they were still expected to open as scheduled, said schools spokeswoman Felicia Cosby.
In Boston, some schools had tree limbs to clear off their grounds after the storm, but none had prolonged power outages.
Schools are expected to open Sept. 8 as scheduled, said school spokesman Matt Wilder.
"We will be ready to go," he said.
Contributing to this report were Associated Press writers Laura Crimaldi in Providence, R.I., Stephanie Reitz in Hartford, Conn., Sarah Brumfield in Baltimore, Md., Zinie Chen Sampson in Richmond, Va., and AP Education Writer Justin Pope.