With the specter of an H1N1 flu resurgence this fall -- and 55 million students preparing to return to the nation's classrooms -- federal officials on Friday released new guidelines for schools that address how to keep the flu from spreading and describe under which circumstances they should consider closing.

Good hygiene is key, they say: Students and staff need to wash their hands frequently, and cough and sneeze into a tissue or shirt sleeve. Sick kids and teachers should be given protective gear and isolate themselves from the rest of the school until they can go home.

Once home, they need to stay put until 24 hours after their fever breaks -- a departure from the Centers for Disease Control and Prevention's (CDC) previous guidelines, which recommended a seven-day seclusion period.

"We absolutely must continue to make prevention our collective business," U.S. Secretary of Education Arne Duncan said.

Vaccination also is strongly encouraged. An H1N1 flu shot is under development, said U.S. Health and Human Services Secretary Kathleen Sebelius, and should be available in mid-October. It likely will require two doses, given about three weeks apart.

The bottom line, federal officials say, is that K-12 schools need to try to stay open. The H1N1 flu is better understood than it was last spring, when it was feared to be more dangerous. And the social costs of letting class out are high, triggering a ripple effect in communities.

More children are unsupervised. Parents, including those who are health care workers, miss work.

"The decision to close schools is a local one and it is one of the most challenging for any school leader," said Janet Napolitano, U.S. Secretary of Homeland Security.

The one exception to staying open, said CDC Director Tom Frieden, is if a school has a substantial number of high-risk students, such as those with cerebral palsy or diabetes, or who are pregnant. Those with underlying medical conditions who start to show flu-like symptoms should promptly seek medical treatment.

While the hope is that no school will have to close, some inevitably will, Duncan said. Educators need to start devising home-schooling plans that use the telephone and Internet, whether it's for just a few students or the entire school. Likewise, parents need to have a contingency plan in the event their children fall ill and have to stay at home.

College-age students are also at high risk for getting the H1N1 flu, public officials say. Not only are they in classes together, but they also may live in dormitories and have roommates. The CDC is continuing to develop its guidelines for this group and is expected to release them Aug. 23.

Frieden said if the H1N1 flu mutates and becomes more deadly, the CDC's guidelines will change. Schools will be advised to actively screen students for the flu as they walk through the doors each day. Those with underlying medical conditions will be told to stay at home. And the distances between students will be increased. Joint classes, for example, will not be held.

"This is an evolving issue," Napolitano said. "Everything is not cut in stone, so our strength and flexibility is going to be very important as we work together to reduce the transmission of this flu."