The opposite of "high" is "low," correct? So when a parent says their child is "high-functioning," they're saying other children are "low-functioning." Even if they don't think that's what they're saying, that's precisely what they're saying.
But what even determines high-functioning vs. low? Having a savant-like skill? Verbal acuity? Passing as "normal"?
My sense is that it's that last one, that the term "high-functioning" is used by parents who feel their kids are close to "normal." Their kids can pass. Maybe it gives these parents some comfort to use that label. I guess so, since I see it used out of context all the time. For example, some parents sign their emails, "Parent to Billy, high-functioning autism." Ugh.
Like some others, I felt that the DSM-V's elimination of the sub-types Aspergers, PDD-NOS, ASD, etc. was, in some regards, problematic. In certain situations, these labels help add context; they are useful in understanding the strengths and challenges one might face.
The APA must have realized this problem, because the latest iteration includes levels of severity for ASD, from Level 1 to Level 3. These, however, are clinical distinctions, helpful in some situations but not in general conversation.
Which leads me to this: maybe all these labels are just problematic to begin with. Maybe the term "autistic" is sufficient. Or maybe even that's too much, I don't know.
So how about this? If you do feel the need to help someone understand what's going on, maybe just talk about the specific issues, and leave the labels at the door.