Mindful of the Florida shooting, administrators supplied Jennings Senior High School in Jennings, Mo., with portable metal detectors and wands to search for weapons at the championship basketball game being held Friday night, said Rhonda Key, the principal. Dr. Key said this would not be permanent. “We’re just doing this because it’s so fresh,” she said.

“You don’t want to scare them,” Dr. Key said. “But when you talk about these things, you must also say there is a real world out there.”

Some districts sent messages on Friday that parents found disconcerting. “We assure you that all rumors are being thoroughly investigated,” read one from the administration of the Haldane Central School District in Putnam County, N.Y. “At this moment, there is no credible threat to the district.”

“Needless to say, I am freaking out,” wrote one parent on Facebook.

At Da Vinci Arts Middle School in Portland, Ore., a drama teacher, Nicole Accuardi, took a different tack after listening to the fear and sadness expressed by many of her students.

During the day on Thursday, her 150 or so drama students cut out paper hearts and stuffed them in their pockets. At a routine end-of-day assembly, on Ms. Accuardi’s cue, they stood up, threw the hearts in the air and sang “You Are My Sunshine” to the surprise of the other teachers and students. Many joined in.

Many schools and districts have developed protocols for handling school shootings since 20-year-old Adam Lanza shot and killed 20 students and six adults at Sandy Hook Elementary School in Newtown, Conn., in 2012.