Washington DC - An Edu-Vacation

I have 2 kids who are going to be sophomores and one that is entering 8th grade.  As freshman, my 2 older kids were offered an educational trip with their school to visit Washington DC and tour the capitol and stuff like that.  The school wanted $3,500....  PER KID.  Well since we almost never spend that much on a family vacation, much less a trip for 2, my wife got to work figuring out how we could DC frugally.  The first thing we would need is cheap airfare, since we live in southern California, or none of this would work.