By mainstreamed you mean what?
Regular public school, regular classes. My high school had (counts) probably 3 or 4 students, counting me, with a significant physical disability (learning and psychological disabilities are important too, and can qualify as severe disabilities, but I'll leave those aside for the moment). Depending on how you define severe, at least 2, maybe 3, of us were severely disabled. Probably the "most severely disabled" student had CP - cerebral palsy. He used an electric wheelchair, and couldn't write or speak or really do much at all that needed fine motor control - he used a synthesizer to speak, and could only operate that by poking a special keyboard with a stick attached to a baseball hat he wore. He had an aid, and he used a computer to do all of his assignments and so on, but all of his classes were honors classes with the non-disabled students, he was involved with NHS and the student council and so on (in various officer positions, even), and went on to an excellent college. (And balanced all of this with being a Paralympian, to boot!)
I could give other examples, both at my high school, and people I've met later in life, but that's a pretty dramatic example right there. Actually, now that I think of it, all of the 3-4 students I mentioned above were in most or all honors-level classes, and all of us graduated in the top 3% of our respective classes. So to say that, "In the class we learnt that if an individual is severly disabled, they cannot go to a public school, they cannot be mainstreamed." means that either the teacher doesn't know what they're talking about, or the lesson was misunderstood. I think it's sad and frustrating - but quite indicative of where our society is right now - that education majors are still learning things like this, *especially* in a class that is supposedly about inclusion.
I'll close with a quote by a major figure in the disabled rights movement, Cass Ivins: "Special. It's such a pretty word, isn't it? But what it means is segregation."