It depnends on your school year. Where we live, school still starts after Labor Day. Because so many schools start in late August, we have found that the last week in August, while still hot as [bleep], has much smaller crowds than the rest of the summer. Similarly, if your schools start early but end around May 20, as I know some do, the week before Memorial Day is not bad.
Otherwise, it's trying to find a break that lines up differently than many other schools. My high school had the entire week of President's Day off many years ago, and the crowds weren't bad. Spring break is handled differently in different places. I've noted that the week before Easter is more crowded than the week after. If your school's holiday break lasts longer than others, you may have the first week in January free, which generally has lower crowds.
But might I offer another suggestion. If you can't work it out to go a less crowded time, change your expectations and the focus of your trip. You are WDW veterans, so you don't have to hit every attraction every trip. I went at Xmas about 5 years ago, and we decided we'd just spend a lot of time enjoying the decorations, seeing the special shows and parades (Holidays Around the World, etc.), looking for hidden Mickeys, enjoying quiet spots in the parks, etc. Each day we'd get fastpasses for a few major attractions, see shows, and selectively hit rides that had smaller lines at the time or were fast-loaders (TTA, JIYI, etc.). We didn't experience nearly as many attractions as we do on most trips, but we still had a great time. And when the crowds just got so bad that we needed a break, we would head to a hotel and look at the decorations there.