It is not in the best interests of society as a whole to exclude people like that. One fairly obvious reason is that even more capable children of such parents will also be, by the examples they see around them, cut off from mainstream society, Another is that idleness can create other problems,such as criminal behavior.
The problem with discussing education is that everyone has been through the school system and therefore considers himself an expert on it. I’m not at all disparaging direct experience ... but ... if anyone is really interested in this vital topic they really should do some information-gathering. Please go to YouTube and enter “Marva Collins” in the search box. Follow that with “Sig Engelmann”. For those of a reading disposition, get ED Hirsch’s Cultural Literacy.
Okay then do this four years of skills education hell I would do this for any student who is likely to drop out over being unable to do some requirements or who isn't motivated to do classes not of interest to them and they are not likely to use is this ... expose them to lots of trades and work doing a week or two of each area and talking about the work then focus on what they want to do. If someone is of low intelligence but they like doing work outside in landscaping why not focus on that sort of work for a couple years including work experience for credit and give them at the end of Vocational Diploma in Landscaping. And one could have them do some side classes in Driving, Small Equipment Maintenance and other classes they show a talent for. And do this for lots of trades and work where college isn't really needed and can be taught in four years. It would be better than having students drop out or fail to graduate with a standard diploma. I will note in much of the world a lot of people work in their teens and build a career skills set by hands on work its better to me than four years of students who may be unmotivated and bored or don't see classes as being valuable.
Some of this could be workable, but your basic assumption seems to be that the trades and etc do not require much intelligence, which is untrue.
It would be largely based on their interests and aptitudes if your a moron well maybe being a cashier or stocking shelves or other work might be all your trainable to do likewise smart kids who just want to work could enter higher end work demanding more intelligence and/or talent for the work. I still would argue for true low income people just sticking them on welfare might be the best thing and leave it at that they really won't be employable the same for the more extreme disabled person.
Im glad you said 'might'. It's really not a good idea to provide free ecological niches for organisms that reproduce.
Learn a moderate paying trade skill. Work for 10 years and save money. Use money to start a business that will employ other skilled tradespeople who want to work to save money to start their own businesses... This is what we should be teaching the majority of kids. Instead we're teaching them to work their hole life making rich asshats richer. It shouldn't be a surprise when a portion of them are like 'F that, I want my own thing'. Then they go off without any idea how to make their own thing because the education system is rigged by folks that don't want them to do that.
By the way I'm not saying these people are bad people in the 50's most would go work on an assembly line in a factory or do some work like farm laborer or other useful employment however its not then and the pool of jobs who are low aptitude are going fast if they aren't gone already.
That is the problem our elites have created; by again externalizing the true cost of their short sighted economic pursuits, they have been destroying the very economic engine that "raised all boats" in the US. Best explained by the instruction, "Don't psss in your own soup"...in this case, meaning one's own country.
To be honest, it has to come from the home; unengaged parents have unengaged children. Furthermore, I do not even see why it should be a goal to make each and every teenager "college ready" because not every personality is suited for such environment. I would state that a good way of improving student performance is legalising child labour so that students unsuitable for education can start their lives and careers in time.