When in March is your break? With the late Easter next year, I think March probably won't be as super-crowded as closer to Easter. I'd go during break rather than pulling out of school, but as you see many do that.
As to swimming -- depends where you live. I'm in the northeast and my DD has enjoyed the WDW pools in December and February, even when we were the only ones in the pool area (because everyone else was bundled in jeans and jackets). If you are from a warmer climate, March may feel cool; though kids tend to enjoy a pool anytime.