Federal law states that in the U.S. every child has a right to a free and appropriate education no matter if the child is gifted & talented, average, or below average. Some children will cost more to educate than others.
There is an illusion that kids come with their abilities and needs stamped on their foreheads. As you have stated, we educate students with a huge range of intellectual, emotional, physical, linguistic, and economic needs. In special education, these needs are defined by arbitrary cut-offs and definitions. The premise is that these categories can be used as predictors of education costs to be incurred by specific disability and need.