Teaching on this 9/11 was an exercise in pivoting: talking about the terrorist events 24 years ago , as well as the events of 24 hours ago .

"People no longer talk about their differences," Rob Wills, a teacher at Cardinal O'Hara High School in Springfield, Pennsylvania, said. "People right away go to fighting."

Wills has taught for nearly three decades. Across the hallway on the third floor of the Delaware County school, Marisa Linsky was also hard at work.

Linsky has taught at Cardinal O'Hara for four years. Both teachers — like so many others — are adjusting history and government class lesson plans.

Wills was at O'Hara 24 years ago.

"I remember it vividly, yes," he said. "And then all panic breaks loose. Parents are pulling up here in the front, pulling their kids out of h

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