By Allison Ross Palm Beach Post Staff Writer Updated: 6:15 a.m. Wednesday, Sept. 7, 2011Posted: 9:05 p.m. Tuesday, Sept. 6, 2011WELLINGTON — Teacher Stu Greydanus still has physics textbooks in his room at Palm Beach Central High that use the height of the World Trade Center as the basis for questions about air resistance, velocity and gravity.But the students in his classes this year really don't remember a time when those twin towers were standing; his sophomores this year were in kindergarten a decade ago."For the freshmen, they don't remember where they were when 9/11 happened," Greydanus said.It's a bit of a disconcerting feeling that an event that forever will be part of his and his fellow teachers' memories is something of which his students have no recollection.As the 10th anniversary of 9/11 approaches, Palm Beach County schools are holding memorials, planning moments of silence, conducting concerts, planting trees and sharing stories from survivors, family members and first responders.They're also grappling with the question about how best to teach students about the momentous events of Sept. 11, 2001.It's a question that's been ongoing since the towers fell, and one whose answer has shifted as time passed and dialogue about the event has changed .Kaylynn Knapp, a social studies teacher at Palm Beach Central High, said one of the biggest issues in teaching about 9/11 is determining what students know and don't know.When she asks some of her students about 9/11, she said some students believe that then-President George W. Bush had the buildings blown up, or that it was "when Afghanistan attacked the U.S." or "when Muslims declared war.""We're having to start from ground zero, and I don't mean that as a pun," Knapp said. "You have to unteach some of the misconceptions."Knapp said she likes to discuss with students the cost of the effects of 9/11, such as the loss of certain civil liberties or how some people might be dealing with health problems in the aftermath."My students are surprised when I tell them that we used to be able to get on an airplane without taking off our shoes or belt or coat," she said.Joe Cominio, a civics teacher at H.L. Watkins Middle School in Palm Beach Gardens, said he tries to blend lessons of patriotism and tolerance into the teaching of 9/11.This year, he is having his students interview parents or other people who remember where they were on Sept. 11, 2001. He is also planning a schoolwide event to raise an American flag that was sent to the school by a soldier fighting in Afghanistan.Neither Florida nor the Palm Beach County School District has a set curriculum for teaching that topic, although they have provided a number of resources for teachers."For the most part, if teachers include 9/11, it's a decision they're making," said Diana Hess, a professor of curriculum and instruction at the University of Wisconsin-Madison who is studying how curriculum and textbooks address 9/11.Hess and fellow researcher Jeremy Stoddard, an associate professor at the College of William & Mary, found that the majority of social studies and history textbooks available today cover 9/11, but overall, the coverage was wanting.They also found that the material covered in the textbooks changed as time passed."Initially, it was heroism, nationalistic themes, for the first textbook editions after 9/11," Stoddard said. "The textbooks talked about the heroism of the day and had a lot of inclusion of personal stories." But he said that, in more recent versions, some of the personal stories and tales of heroism have been taken out to make room for things that have happened since.Later versions also ventured to start asking students to deliberate on the effects of terrorism and some of the issues surrounding the attacks.And the textbooks, Stoddard noted, don't spend a lot of time discussing the actual events of 9/11, such as the number of people who died or exactly what happened.Deciding how to teach 9/11 can be difficult, said Laurie Cotton, the school district's secondary curriculum social studies program planner."This is contemporary history," she said. "There are people who remember it well and have very emotional attitudes of how it should be addressed."But despite the impact 9/11 has had, not all teachers cover it in history or social studies classes."There's not any way around it. September 11 became our history that morning," said Ryan Wells, an advanced placement human geography teacher at Forest Hill High. "As each year passes they have less and less memories of what happened that day. It's our job to teach them about it."