From the statutory text, it seems that when Congress passed the IDEA, it wanted disabled students to be receive the same (or equivalent) education as nondisabled students. At least until they were 21. At that point, they're on their own.
Hawaii offers a GED program. For lots of people. But it decides not to follow the IDEA for 20 and 21 year old students.
I understand -- even if I might not necessarily agree -- with that decision. Hawaii wants a cheap and easy GED program. It doesn't want to have to prepare IEPs and deal with all the hassles (and expense) involved in trying to educate disabled people.
But the IDEA has other ideas. If you give an educational program to 20 and 21 year olds that looks like a secondary school program -- e.g., one that essentially gives you a "high school diploma" -- you've got to give that program to disabled people as well.
The Ninth Circuit agrees.
So from Hawaii's perspective, its GED program just got more expensive. But from the perspective of a disabled Hawaii student trying to get a GED, doing so just became a bit more feasible.