Sure they can! I can tell you right now when it will be. (aside from a few renegade districts like Canton and whatever the other one was.) And while there is no law, essentially all of Texas has Spring Break the same week in March. There will be a few renegades here and there, but 95% of the state has Spring Break the same week.
Cruiselines can make an educated guess based on data, but I was trying to state that there is not a set week in Texas for the entire state to have Spring Break, and that neighboring districts can vary from one another.
Dallas ISD was the other "renegade" district I had shared previously (a rather large district with 159,000+ students). Their Spring Break is actually the week of March 15 in both 2012 and
2013. March 11-15, 2013.
Disney, you messed up! Spring Break is an excellent time for an 8 night cruise. But you scheduled the 6 night cruise out of Galveston for Texas' Spring Break, and the 8 night cruise for the following week. Maybe you thought Texas' Spring Break was March 15?
Out of curiousity, I checked a few others based on
student population to see their calendars (linked):
Houston ISD with 200,000+ students is also another renegade district with Spring Break March 11-15, 2013.
Cy-Fair, the third largest district in Texas with 92,000+: March 11-15, 2013
Northside ISD, fourth largest with 82,000+, March 11-15, 2013
Austin ISD, fifth largest with 82,000+, March 11-15, 2013
Two other schools that are the week of March 15 (2013) are Texas A&M and University of Texas at Austin, but of course those aren't K-12.
Could it be possible that
DCL looked at the five
largest K-12 school districts in Texas and the two
largest universities in Texas (A&M and
UT), saw that those institutions all have Spring Break 2013 as the week of March 11-15, and made a data-driven decision?

That's ~550,000 K-12 students just from those five districts plus 100,000+ college students from those two universities. Or maybe it just came down to port space.
Some districts are already working on their 2013-2014 calendars.
Austin ISD tries to align with UT Austin (March 17-21, 2014).
Examples for varying 2012 Spring Breaks of two districts in Texas:
Dallas ISD Spring Break March 12 - 16
Canton ISD Spring Break April 2 - 6
These two district's boundaries are less than 100 miles from each other.
Disney can make a great educated guess, especially with the K-12 public school
assessment calendar having STARR dates out in future years, and especially with the college calendars. Spring Break varies from district to district, from college to college, and state to state. Due to the testing calendar of public K-12 schools in Texas, Spring Break has certain weeks it will not exist. This year, 4th, 5th, 7th and 8th graders have STARR the last week of March, along with some high school courses, so no district would likely have Spring Break that week.
I'm sorry, but when I read this thread's title and first post, especially the sentence "Maybe you thought Texas' Spring Break was March 15?", I took it to mean that Texas had a set Spring Break for the state of K-12 schools. Then reading about the law from a different poster, my curiosity got the best of me to double-check that since I hadn't heard it. Just looking at only the top five districts with 550,000 students having Spring Break the week of March 15 in 2013, and less than 4.5 million K-12 public school students in Texas, I think that's a pretty good percentage of students IMHO that may have played a part in Disney placing an 8-nt cruise with holiday pricing the week of March 15. Over 10% of the state's K-12 students just from five districts have Spring Break the week of March 15, 2013 - without even looking at the ten other Texas
districts with 50,000+ students each. I didn't mean to ruffle any feathers, just share some data that might shed insight to a possible reason for the 8-night cruise the week of March 15 and that Texas doesn't have a set Spring Break date.