Some students, with more severe disabilities, are more set apart. Those with significant cognitive disabilities might be in a life skills class (although they still take electives and lunch with other students), and those with severe emotional disturbances or who have proven to be repeatedly violent may be put at Whaley or other specialized programs. But federal law requires the least restrictive environment, and for most kids who have relatively minor disabilities, they are mainstreamed into a more inclusive environment. Which is great for many but not every student. To move a student to a more restrictive environment is a very cumbersome process, particularly if the parent objects. 

Other districts use more self contained classes but ASD is big on inclusion. For many that's good, but it isn't beneficial to everyone who has a disability or sometimes even other students. It just depends.