I know what you mean about not being able to take the kids out of school. We have the same situation. So we've gone in summer for our last few trips. Here's our experience. It's hot and humid (prepare yourself mentally for this otherwise it will hit you like a brick). It's rainy. Meaning it often rains for more than just 10 minutes. Sometimes it rains for hours. Be prepared for this too. Tell yourself that if it pours, you don't care, you'd rather be wet in WDW than dry in your hometown. Bring umbrellas and ponchos. You will need them. It's crowded, but since Fastpass came into existence, it's much better. We've done June and July, and didn't find there to be any difference in weather or crowds. The big plus to going in the summer is how late the parks stay open. You can really do alot that way. Get to the parks early, break in the afternoon, stay at the parks late. And if there's an E-Night - do that for sure. You can ride all the biggies over and over again with little to no lines.
We're going at the end of the summer next time, and it's considered off-season. I'm bummed because I really do love the summer hours. We're simply going later due to costs.