I agree with a couple of people above that the first week of your summer break or the last week, whichever is most likely to be less crowded, might be your best bet. Check the calendars for AP blackouts dates and try to go when the SoCal passes are blocked. Go midweek, with one weekend evening (eg Sun- Wed or Tue-fri).
We usually go during spring break, and yes, it's crowded but with a strategy, it's still incredibly fun! But I wouldn't recommend Spring Break in 2015 because of the date of Easter I think it will be more crowded than typical spring breaks. But that may depend on when your individual school does spring break. Best to avoid DL the weeks before and after Easter.
Plan to be in the parks at least 2 full days (for both morning low-crowd time and evening entertainment time) and at least 2 partial days (for either morning time or evening time) in other words, at least 4 park days total. This will give you the chance to see almost everything, including shows, have time for repeat rides, and most importantly, have time to relax. 3 full days might be enough, but you'll be exhausted. On the other hand 5 park days wouldn't hurt