My wife and mother-in-law are special education teachers. My wife is more involved with ID (intellectually disabled) kids, while my mother-in-law is more involved with LD (learning disabled) kids. They both have experience with the entire range of special ed though, since special educators tend to get bounced around to cover the numbers as students progress between schools - so I get to hear about this stuff all the time.
ADHD is no doubt a real thing, which has gotten a bad reputation because of over-diagnosis and over-medication in the last 10-20 years. I think that people in this thread questioning it have just never been in a classroom with children that are actually affected by it. All kids are spastic and would rather do fun/crazy stuff than sit down and learn. There is a big difference between "all kids" and real ADHD.
Autism of any form should not be lumped in with ADHD. Even Asperger syndrome, which is relatively mild compared to other ASDs and forms of ID, can make learning much more difficult for kids. I don't think that it's in our best interest to tell kids with Asperger's or dyslexia to just "suck it up" when some basic accommodations can help them to succeed as well as any other student. A majority of your ASD and LD kids don't even end up in separate classrooms from "normal" kids. They sit in regular ed classrooms that have 2 teachers. The 2nd teacher is SPED and helps with accommodations for either behavior or learning differences. This could include things like oral exams (instead of written), prompts to keep the kid on task, etc.
Even your more profoundly affected ASD and ID kids are worth teaching. They aren't going to go to college or probably even earn a standard high school diploma, but proper training at school can make a big impact on their lives. They can learn practical skills that we take for granted (ex. personal hygiene, paying bills, cooking, laundry, communicating with people, basic job skills, etc.). Providing them with this education and training can allow them to be much more independent as adults and keep them out of adult care facilities when their parents die or can no longer take care of them.
I was in the gifted programs at school. I was one of those kids that was bored most of the time at school. I know that we could do a lot more for gifted kids, but the money just isn't there to offer full-time gifted education at most schools. Parents need to challenge their gifted kids at home and push them towards activities that exercise their minds. Parents can also advocate for their gifted children to get accommodations at school like skipping grade levels in certain courses or creating relationships with local colleges so their children can dual enroll and earn high school credit for college level courses.
