Hi colbybug, your question is not uncommon at this time of year.

And as you've seen in many of the responses above, Spring Break varies from school district to school district as well as from the neighboring States.
However, there is a clear answer to the question: "When does Disney expect the largest concentration of Spring Break visitors?" And the answer can be found by looking at the Annual Passholder Blockout Dates.
AP BLOCKOUT DATES
Southern California Select APs: April 3-19
Southern California APs: April 4-18
Deluxe APs: April 10, 11
In other words, Disney blocks out the local passholders on the days they expect larger crowds from out-of-town. April 4-18 includes the week before and the week after Easter (April 12).
Regardless, there will be guests on Spring Break in March, as well as in April around these dates. But the largest numbers of Spring Breakers will be visiting during those two weeks. I hope that helps your trip planning!