A photo Tweeted by Assemblyman Alec Brook-Krasny shows Brooklyn's I.S. 289 surrounded by water this morning.

New York City schools will be closed for an unprecedented third straight day Wednesday as the city continues its clean-up and recovery efforts after Hurricane Sandy swept through the region Monday night.

Some parts of the city remain underwater and it is likely to be many days before the city’s subway system is fully operational again, Mayor Bloomberg announced today at his first press conference since the storm ended. Residents displaced from their homes continue to be housed at shelters inside 76 school buildings, as well, suggesting that it could be some time before schools can reopen.

Some of the shelters lost power Monday night and are now operating using generators, Bloomberg said today.

It is becoming clear that at least some schools have suffered more extensive damage. A video taken this morning showed what appeared to be an explosion inside John Dewey High School, located just blocks from the water in eastern Brooklyn. The fire started in Dewey’s basement, according to a Twitter feed that reports FDNY alerts, and efforts to assess the damage are underway now, according to a Department of Education spokeswoman.

State Assemblyman Alec Brook-Krasny published a picture this morning of I.S. 239 Mark Twain School, located blocks away from Dewey also on the shore, that showed water lapping up around the building. Other photos we’ve seen on Twitter today show smashed-up school buses at a Brooklyn depot; a tree down outside the building Coney Island Preparatory Charter School shares with I.S. 303; and a large tree down outside Brooklyn Technical High School, which is being used as a shelter.

(Add links in the comments section to school-related photos you have seen or taken of Sandy’s aftermath.)

Several readers have asked us whether the days off will be rescheduled. The answer is yes, probably, but no official word has come down yet.

The state’s guidelines say that school days missed for inclement weather or other emergencies must be made up. According to the rules, school days off associated with religious holidays are fair game for rescheduling, but weekends and legal holidays are not. And the state education commissioner has the right to forgive up to five missed days, but only if they happen so late in the year that they cannot be made up.

The Department of Education usually sets aside some school days for snow make-up days; last year, two days at the end of the school year were turned into training days for teachers to prepare for new learning standards.

City schools are supposed to be closed a week from today for Election Day, when many will be used as polling sites.