I have a quick comment to make for those who are in school or at university. I went to World 2003 and 2005 while I was at university. Both times I ended up missing some school days, and both times I approached my professors beforehand telling them that going to this competition was extremely important to me, and what could be done? Both times my professors worked with me. I had crap tons of make-up work to do, but I was able to go. Also, at Worlds 2007 there were a lot of people from the California Universities who were working on assignments that had to be turned in even while they were at the competition.
For those in school or university yes it will be harder for you to go than most. You will have to make-up work and/or change your schedule. But, it can be done. Don't tell yourself you can't go until you talk to your professors. There may even be some consequences to your grade for lost attendance and such, but if you tell your professors that this is important to you, then there is a likely chance that you will be able to work out some sort of deal with him or her.