I read this thread with interest because not only do I have 3 school aged children, but DH is a school teacher. We can't pull the boys from school because DH can't take time off during the school year. We are limited to going during school breaks.
For the past 2 years, we've gone in late August, and I will tell you that it's hot. Last year, I think my littlest guy, who was turning 5, got heat poisoning (if there's such a thing). In any event, he spent the last day of our vacation in our room with my mom, getting sick.
We decided not to go in August anymore. We're not taking a family Disney trip this year, although DH and I will be there for a long weekend in September (he's taking one day off from work). We're planning our next family trip in 2008 and we will either go in February or April.
We get a week off in February (mid-winter recess) and I've heard that the parks aren't too crowded then, plus it's not as hot. Next year, we will get 2 spring recesses - one around Easter (March) and another around Passover (April). We're thinking that since not many people will be getting a week off for Passover, that might be an even better time to go. It'll be warmer than February and the crowds shouldn't be too bad.
I think that if you're creative, you can often work with the school calendar.