We took our kids out of school a few times for our Disney trips. Most recently, we took them out at the end of September 2005 for a week. My oldest was in 5th grade and my youngest in 1st. We surprised them, so they had NO idea about the trip. This was a GREAT thing, because they remained focused on school right up until we told them we were going to WDW the night before we left. I spoke with each of their teachers in private, told them the boys didn't know about the trip, and they were more than happy to get homework together and to me without the boys knowing. Our district requires that we fill out an extended absence form before a trip, so I did that without the kids knowing too.
We are planning another trip in May 2007. There will only be 3 weeks of school left when we go. My oldest is in 6th grade, and the youngest is in 2nd. I did check with my 6th grader's teacher to be sure there was no district testing going on, before I made our plans. I figure with only 3 weeks of school remaining, there won't be a ton that either one of them will miss. The only thing I found out AFTER I booked was that my 6th grader's middle school band concert will be held during our trip. There's nothing I could do about it now, since I already bought airfare. What's done is done. I haven't told him yet, because they don't know when we are going. They KNOW we're going, but they THINK we're going in June.
My only advice is to try to keep the trip a surprise if your kids are obsessive about things like mine are. My oldest is especially like this and will totally lose focus a few weeks before the trip. If he doesn't know about it, he can't obsess over it. It works for us!
Have fun!