I have 3 main points when it comes to autism. Autism is a spectrum and each case has different severity on the person affected. Light autism (Autism Spectrum Disorder or formerly Asperger's Syndrome), moderate autism, and severe autism. Light autism is what I am. We function differently but we can for the most part be as intelligent if not more than than the average person. People theorize that Albert Einstein was an undiagnosed light autistic person. As for moderate autism, that is when someone can understand basic issues and terms and may be able to utter some brief phrases and do things without assistance, but will need some form of support for the remainder of their life. Severe autism results in a non-verbal and are dysgraphic. These people will need life-long support in the same case as an elderly person with dementia. They cannot do any form of basic self-care and must be cleaned, assisted in the restroom, and hand fed.

Now, I want to be very clear here in that what I just said above is a very broad scope and each diagnosed case of autism is going to be different in various ways. A severely autistic man may be able to feed himself if handed food and have certain other skills, but may have trouble with the other mentioned issues of self-care. Meanwhile someone who is light in autism isn't guaranteed to just be "weird in person". I have epilepsy myself on top of that. Also, neurological conditions frequently overlap in the case of autistic people. I was not the only autistic and epileptic person in that school.

Severely autistic children and men may not ever integrate into society and they should just have a caretaker during their time in school without being disturbed too much outside of some if any activities they may be able to derive enjoyment from. I'm talking about non-verbal people who cannot communicate through text. I mentioned in another comment that the severely autistic children were frequently given assignments to try and teach them even kindergarten level speech/writing but it never amounted to anything for anyone there and frequently resulted in even the severely autistic children getting upset, then getting taken out and they are unlikely to understand what they did wrong when the staff were the major aggressors in almost all scenarios.

You do not see severely autistic people in school often because they are frequently sent to schools like my own and kept there until I presume they "graduate". It's not uncommon for them to be sent as early as first grade or even kindergarten if they are completely incapable of doing assignments. They are more common than you think and you can easily populate 1-2 classrooms per school district with varying degrees of autistic school children. Some of them get sent away but you're basically guaranteed to not participate in a regular school with severe autism.

Moderately autistic people are frequently there for a majority of their years, but may be able to pass kindergarten and first grade with some enhanced difficulty. They are like severely autistic people but are more capable with basic functions and aren't necessarily non-verbal or dysgraphic. However it's always going to be an uphill battle trying to teach them anything complex or having them deal with complicated scenarios. It is likely in most areas you do not see anyone moderately autistic as they do not tend to last long and while they don't stand out nearly as much as severe autism, they do still stand out a fair amount and they will struggle to keep pace with the other children and failing very early elementary classes is a quick ticket to a school like the one I mentioned.

Finally, people like me go to these schools only if we have developmental or behavioral trouble or the parents recommend it. Many high-functioning autistic children had difficult times in school either not understanding right from wrong or being influenced by someone bad. I had issues with fatigue to my epilepsy and sleep schedules, and I inadvertently fell asleep in class once which caused the principal to appear in person and physically pull me out of my chair which I had responded violently. That ended up getting me put in the charting school after a discussion with him and my mother. I entered the boarding school not knowing what I did wrong since a stranger was physically pulling me without warning in the primary school system. Keep in mind, I was not familiar with the principal as I had just entered middle school (I was only there for a few months). So some man came in from nowhere and started dragging me out of my classroom, and I didn't know how to properly act. My mother put me in the boarding school not knowing how bad the environment was and I began to hate school and I never felt like I had anyone to look up upon as my older neurotypical brother frequently injured me for being "stupid" (he has a much more mellow personality in adulthood and felt remorse for his actions some 15+ years later knowing my autism diagnosis) so I often looked to entertainment media (aka violence in a lot of it) to try and understand how people acted.

Only when my mother got internet for my brother did I learn exactly how other people acted, since during school I did not particularly relate to adults or other students that well, but people on the internet treated me like an equal without regard to my disability. It was only with social media that I grew as a person and learned I wasn't a failure or beyond hope. I just got into the internet far too late as rural america lacked internet for far longer than people in suburban and urban communities. Schools did not let you use the internet as far as I remember as it served no purpose for education.

One final parting note is that I believe that autism in general is horribly misunderstood. I said severe autistic people are looked upon like animals in the other comment, but that's only the base of it. Autism is misunderstood and so many false stereotypes exist about people like us. One popular example is that people like me are inherently shy because we don't look people in the eyes. This is one of the bigger stereotypes peddled. Autistic people can be shy, but we aren't inherently afraid of neurotypicals just because we don't look you in the eyes. And I can be quite mean if I feel someone is coddling me for that or treating me any less than the person I am due to these stereotypes.

I hope this answers in depth all of your questions. I believe that severe and moderate autistic people should get the support they need without being haggled by staff in these boarding schools, and I also believe that whatever approach that these staff are currently using are ineffective and result in worsened behavior or traumatic memories that negatively effect high functioning autistic children.