You know, I’m not sure. That visit with the Mennonites was a long time ago, we’re talking at least 5 or 6 years, so I can’t remember. I believe it is mostly home schooling, and so it may not be university standards, but even if it’s not, GEDs can be gotten without a huge degree of difficulty. Keep in mind, most high school classes, especially basic ones such as earth science, math, and literature, have a place in Amish culture, so they would have at least been introduced to it, if not fully schooled.
Amish have no more difficult in learning then anyone else, and I’d expect that the culture may give them some better powers of concentration, so I’d expect that even if their education was lacking, taking classes for a short period and obtaining a GRE would not be all that difficult. And I would not find it surprising if the majority of parents supported their children if they were to attempt this, especially as this period of adolescence is expected. And as @judochop had an Amish friend in college, it is obviously possible.
Sorry I can’t remember much more, but hope I helped.