I don't like the terms "low functioning" or "high functioning" in relation to autism. For one, it is hard to quantify a meaningful measure when the spectrum has broad as well as deep criteria for diagnosis. Is a child who is friendly and interacts well with adults, has poor or no language capability, many sensory sensitivities that limit ability to be in high stimulus environments, and can also draw or paint exceptionally well, "high" or "low" functioning? How about a child who communicates with language, is very good at academic work, but has significant problems with social situations, can't write with regular writing implements or play any kind of sports due to apraxia, and has constant and noticeable stims?
When people use this measure, you can see that they want to sort children into "boxes" to make thinking about them easier and less complicated. At best, they see "functioning" as a sort of range of values from 0, unable to function within society at all without assistance, to 100, virtually indistinguishable from "average".
The worst part of these terms as they are used isn't just the assumption of a single measure to apply to a whole range of symptoms, behaviours, and sensory processing disorders. It is the whole concept of "functioning".
Because what people seem to mean by "functioning" is "how well does this child meet our expectations of normal?" and "how well does this child fit in with peers?", which boils down to "How well can this child hide the differences that autism creates, and fake being normal?"
My kids are whole beings. Kitten is more at ease socially, but has little practical language. Monkey has come past being non-verbal, but has a significant speech impediment, and suffers socially. Kitten has more frequent and easily recognized stims, but Monkey has more disruptive stims that are often misunderstood to be deliberately annoying behaviours. Kitten has more violent behaviours towards herself and others, Monkey has crippling anxiety and low self-worth. Some of these things are going to be "treatable" and certainly some of them will change over time. Does a child with "high functioning" autism who later exhibits more unusual and socially awkward behaviours become "low functioning"? Does a child with "low functioning" autism who learns to effectively use language suddenly become "higher functioning"? Autism is a part of brain function, it doesn't go away just because the child learns and grows.