Terms such as high-functioning versus low-functioning children are derogatory and should be avoided.
Reference: Lovaas, O.I. (1996). The UCLA young autism model of service delivery. In C. Maurice, G. Green, & S. Luce (Eds.), Behavioral intervention for young children with autism: A manual for parents and professionals. (pp. 241-248). Austin TX: Pro-Ed.
This was unexpected for a whole lot of reasons, the main one being that Lovaas's style of behavioral treatment for autism is fairly notorious for ignoring the autistic child's inner life entirely, concerning itself only with getting the child to engage in socially-acceptable speech and play, while "extinguishing" whatever behaviors they deem aberrant. Indeed, concern for the child's feelings and well-being are entirely absent from this 1985 paper, in which he advocates ignoring or punishing all behaviors that aren't the desired behavior.
I also found this entry on the Lovaas Institute Blog dealing with language: the blogger is aware of the dehumanizing connotations of the language used to describe behavioral treatment, but rather than do anything to put the focus on the autistic child, and his or her actual needs, he just says (paraphrased): "Well, since I feel love for the kids I work with, and get all teary when they learn something, what I'm doing can't be dehumanizing!" His feelings define the interaction, it seems. Lovely.
Anyway, back to the question of using "high-" or "low-functioning" to describe autistics. As I've mentioned before on this blog, I do not use such terminology because I don't find it terribly descriptive or useful. Autism is characterized by wildly uneven levels of competence in different areas, or even in different parts of one area. Like, you might be amazingly fast with mental math, but unable to grasp the simplest kinds of mathematical reasoning. Or you might have an astonishing vocabulary but struggle with constructing sentences or arranging a paragraph. Or, for a more dramatic example, you might be completely unable to speak, but be able to write or type with great eloquence. The whole concept of "general intelligence," around which IQ is based (and, guess what, in many studies the boundary between high- and low-functioning autism is an IQ score), takes for granted an NT style of cognition. Apart from that, a given person can also vary wildly in ability within a single area over time. Day to day, I range from hyperlexic and able to carry on intellectual conversation to completely mute. With this kind of unpredictability, the same person might be high-functioning in x, y and z domains and also low-functioning in a, b and c domains, or high-functioning in x domain on Tuesday but low-functioning in x on Wednesday. Maybe it's just the Aspie in me, but I need a bit more precision in a classification scheme before I'll agree to use it.