The current plan used down here is something called "inclusion", which is what I believe some of you are referring to with the whole "speds and normies in the same room" thing. This has been shown to rarely, if ever, work. All it really does is stress out the teacher and causes a detriment to educated the regular kids as the sped (or speds; yes, there are schools that will put more than one in a room) causes repeated disruptions. Like someone in the thread said earlier, the teacher can't really teach the rest of the class if they keep having to deal with Jimmy Crayonchewer.
First, you need to separate these kids. It's harsh, but it's true. Those who are not hopeless causes need specialized attention from those who are also properly trained in dealing with their behavioral issues when they either have a meltdown or worse (mom had kids who would throw chairs and desks; others who would literally run off-campus and into the street if not contained). For some of these kids, with enough specialized intervention/attention, you can eventually re-integrate them into the general student population. These would most likely be your high-functioning types, at-risk types, your emotionally/learning disabled. But again, the key word is: properly trained. The average teacher is not equipped to deal with this shit, which is a huge part of the problem.
Now, that's the first issue. Second issue, sometimes the kids are not ever going to be able to re-integrate. Those are the low-functioning kids, your severely mentally disabled types. These kids struggle with more than just social skills, these are the ones who struggle cognitively and physically (your hydrocephaly kids, for example) with basic survival at times. It's again, harsh but true, but people need to stop expecting some kind of school miracle that's going to turn these kids into high-functioners. However, it's not a complete washout - despite the severe handicap, a lot of these kids still have some measure of trainability. You can't teach 'em to take a standardized test but at the very least you could teach them to signal you when they need to use the bathroom so you're not cleaning up shit every couple of hours and it will make everyone's life so much easier. If you have a mid-range type (such as your Downs kids), you can instruct them in some more complex skills, such as setting them up for future vocational efforts.
Your mid-to-severes, though, are not going to make it in the academic setting. They just aren't. No Child Left Behind is a crock of shit. These kids can do things/can be taught things, but everyone's time and money is being wasted trying to teach them things that they just cannot and will not ever grasp, and it just makes things harder for said kids.
Third issue is parental intervention. Sometimes, mom had good parents who were just overwhelmed (the school she was at was somewhat lower-income so some of her kids' parents were just the exhausted single mom types), some of them were the kind of shitheads who never come to IEP meetings and if they do, it's while utterly shitfaced drunk/high/whatever. Nowadays, though, parents actually "parent" even less than ever. School is not your goddamn containment pen. Your little angel can be a nasty little heathen. Without parental support, all the intervention in the world is not going to do the kids a bit of good, because as soon as they go home at the end of the day, there's nothing to reinforce that intervention. It's simple goddamn behaviorism.
To use a horse example, our horsetrainer had an unruly piece of shit that an owner dropped off and some cash for her to 'fix' it and make it show ready. This horse was barely halter-broken, it was 17 hands of pure unbridled murder. But the trainer grit her teeth and busted her ass fixing its behavior and turning it into almost a whole new animal, and even managed to take it to a couple of smaller practice shows where it won some awards. At the end of the paid-for period, the trainer told the owner that they needed to keep up the horse's training by doing XYZ practice/skill reinforcement with it.
Horse goes home, the owner does nothing. Horse reverts back into being a piece of shit. Trainer refused to take him back after that.
Unfortunately, due to federal laws ensuring the rights of a free education to every child, teachers can't say no/reject a child when a parent won't fucking pick up their end of the work.
Fourth issue is structural/governmental. As of right now, there aren't even enough REGULAR teachers to fill schools in this state, let alone ExEd-certified ones. Social workers are so overloaded they burn out before they can do any measure of good. Underfunded public schools don't always have the money for separate classrooms for an ExEd program or said teachers. Adult education/post-school education programs to keep up the reinforcement/intervention are few and far between and also suffer from underfunding. If privatized institutions/programs exist, these are frequently out of reach of those who need them most due to the cost of utilizing them. This isn't even going into the issues of abuse a lot of said places inflict, either.
Basically, special education is there to fill a need, and could potentially work. Would it be nice if we could save every kid (and even adult) and give them opportunities to do things they're capable of, rather than let a lot of speds end up being parasites on an already overburdened system, or leave them vulnerable to other parasites who would take advantage of them for financial or sadistic gain? Sure. But as it stands, most people either don't want to accept the limitations of some kids or don't want to put in the money/effort to actually make that a reality.
(The real ideal is that we wouldn't have anyone being a sped in the first place and everyone has equal capabilities/opportunities, but we all know that's a eugenical pipe dream.)