I just signed up to the "free for individual students and educators" plan using an .ac.uk domain (the UK equivalent of the US' .edu). Perhaps your problem is more of an edge case? What is the TLD you are trying to use?
My wider point is that it's disingenuous to have a bold heading [0] "Free for individual students and educators", and then in the FAQ lower on the page state:
"You're not eligible if...you're a k-12 student or k-12 educator"[0]
I remember this wording irritating me when I was a k-12 student myself. I would get excited at "Free for students!" headings and then be disappointed when the discounts were in fact for higher education only.
It's misleading, really.
AFAIK .edu is used by pretty much all US universities, and very few outside the us. I went to a university that does have a .edu domain (ethz.ch is aliased as eth.edu), but that was a completely undocumented feature. You just had to be in-the-know.