According to their Education FAQs, they have to accept anyone who is currently enrolled in K-12, or pursuing Higher Education.
http://www.apple.com/backtoschool/faq.html
K-12 students are not eligible...check those terms again and you'll see that employees of K-12 institutions, as well as PTA officers, school board officials, and homeschool teachers are eligible. But not students.
(And graduate students are eligible...it all falls under higher education. I've purchased several machines in the EDU store through my grad school.)