Today’s high school seniors were only second-graders a decade ago when two planes crashed into the World Trade Center, another hit the Pentagon, and a fourth crashed in a field in rural Pennsylvania.While most adults have watched the footage again and again, there’s no guarantee children have seen any video from the most tragic event on American soil in over 50 years. Most fourth-graders weren’t even born at the time.“I’ve found most of students don’t know a lot about it,” said Rex Crabtree, a seventh- and eighth-grade social studies teacher at Centertown School. “The ones who do think of it as only happening in New York City.