Our first trip we went the Friday 10 days before Easter Sunday and stayed till the Thursday before Easter. From Friday till Tuesday it wasn't too bad, but by Wednesday it was awful. It just got progressively worse each day. At that time I felt like it was our "one trip to Disney" which added to the pressure of trying to do it all, which didn't happen. Luckily, I conned my husband into a trip last July - it was crowded, but compared to Spring Break it was nothing. It was very very hot, and DH doesn't tolerate heat well, but I bought these things called Cool Neck Ties, which you soak in ice water then tie around your neck - they really work to keep you from getting too hot.
We will definately go in the summer before Spring Break because we can't take the kids out of school either. But, if you go knowing what to expect, and don't try to "do it all" you will be fine.