Victoria Millon bowed her head during a moment of silence at her Miami elementary school on Monday and thought about the 20 children nearly her age who were killed in their classrooms in Connecticut last week.

They wouldn't be able to open their Christmas presents, the 11-year-old thought. And they would never grow up.

"They had their whole life ahead of them," the girl said.

The principal assured Victoria and her classmates they were all safe, and afterward the students talked with their teacher about the shooting and the disturbed man who fired the shots. When someone started to talk about how some of the kids in Newtown, Conn., saw their friends and teacher die, Victoria started to cry.

The horrific shooting that left 26 people dead at Sandy Hook Elementary School on Friday reverberated in the nation's classrooms Monday. Students worried about their safety, teachers faced difficult questions, administrators reviewed security plans. and police responded to reports of new threats. Worried parents considered keeping their children home, only to painfully conclude they could only shelter them so much.

"Ultimately, if this is going to happen ... nowadays, it could happen in a movie theater, at the mall, anywhere," said Lilly Rosell as she nervously surveyed her 7-year-old daughter's Miami elementary school. "It's now about being in the prayer closet a little more often."

At least three schools were on alert in Ohio after threatening comments were made on Facebook and Twitter. In suburban Philadelphia, officers rushed to a high school after security officers mistook a student's umbrella for a gun. In Tampa, Fla., the Hillsborough County Sheriff's Office questioned students after a bullet was found on a school bus.

And in Ridgefield, Conn., swarms of parents picked up their children and police were dispatched to every school after a report of a suspicious person at a nearby train station.

Emily Pambianchi, a third-grade teacher at a private school there, said she and her students sprang into action when a lockdown was announced in response to the threat. Lights went off, blinds were shut, doors were locked and students were ushered into safe areas.

"We read books, talked about holiday traditions and shared ideas about the holidays with one another in the dark," Pambianchi said.

An officer later came and unlocked their door. He was introduced to the children and helped explain the lockdown as a safety procedure, similar to what they do during a fire drill.

Some parents decided not to take their kids to school at all. Camille Lacroix-Moulton said her two children woke up feeling a bit under the weather, so she decided to keep them home. Her daughter is in kindergarten, her son in fifth grade.

"Mainly because of my little one. She just turned 6, and I don't want her to hear about it," the Milford, N.H., mother said. "It wasn't really me thinking, 'Today's the day that something bad's gonna happen to her.' It was more like, a lot of this stuff is going on today. I'm sure a lot of kids know about it, even at her age."

Chicago resident Melissa Tucker said she only sent her children to school after assurances from administrators that extra safety precautions had been taken.

"I was actually going to keep them home today," she said.

One school district in western Pennsylvania went so far as to get a court order allowing armed officers in each of its schools on Monday. The board had recently voted to let officers carry guns but decided to expedite the process after Friday's shooting. The court order affected the Butler Area School District and the South Butler County School District, about 30 miles north of Pittsburgh.

Many schools held a moment of silence and flew flags at half-staff. Teachers and administrators tried to handle the psychological toll, many opting for routine rather than a discussion about the shooting.

Kit Bell, 65, a second-grade teacher at Sunnyside Elementary School in San Francisco, said she didn't talk to her students about the shooting, but that teachers were discussing it among themselves and questioning safety procedures. Continued...