I have seen this trend in my own classes; students with names near the end of the alphabet generally do worse. This reminds me of the classic correlation vs causation problem (my comments at the bottom). So here are my attempts at explaining this phenomenon:
1) Business owners with last names near the end of the alphabet are more likely to be in the back of the yellow pages (or at the end of a bookshelf), more likely to be less successful, and more likely to have less money to spend on their kids education.
2) Whether it's lining up for lunch in elementary school or handing out papers in high school, people with last names near the end of the alphabet may have a more contentious relationship with the educational system.
3) When put in alphabetical order, they may be more likely to be in the back of the class, which has a negative effect on learning.
4) Being at the end of the alphabet puts you later in class registry for college, which could decrease grades, which could decrease income, which could decrease the money spend on children's education.
5) Perhaps last names starting with X, Y and Z are harder to pronounce, which I can imagine has a negative impact on career.
I'm grasping at straws, anyone else have a better explanation?