"the assignment syntax "x=x+1" must be especially confusing to high-school kids"
Many other comments already discussing this, but I have to add this. I don't think that has ever confused more than a handful of high school students because you are almost certainly bringing an understanding of "=" that you acquired at a much higher level of math.
For a lot of students, "=" isn't even what you would consider the "equality" operator... it's the simplify operator. That is:
2 + 2 = ____
has, for them, One and Precisely One answer. (You know this to be true, because your brain has already supplied that answer, and you already know that "3 + 1" is not an acceptable thing to put in that blank, even if a later understanding swoops in to say "Well, yes it is..." You still first knew the One and Precisely One answer.) So it's not the "equality" operator for them.
(I will give this kudo to Common Core, it tries much harder to avoid grinding the "simplify" definition into children's heads than older curricula.)
I don't think they get much collision with the "simplify" operator while programming because the contexts are too different.
I'd also submit that the conception of mathematical statements existing timelessly in some abstract math space would be foreign to them, and it is not hard for them to develop the "intuition" that "x = x + 1" is simply a statement that "after" this statement x will be one greater.
The only people who have trouble with "x = x + 1" are mathematicians trained to a very particular formal level, where they understand the idea of equality more deeply than a high school student but still haven't noticed that operators and formalisms change between branches of math all the time and insisting on your precise definition of an operator hasn't yet inhibited you from understanding the branches of math that redefine it themselves. Granted, no branch of math uses a definition that looks like the imperative programming version (even mathematical formalizations of imperative programming tend not to use = for assignment from what I can recall), but it's still just one more definition in a constellation of definitions, not the uniquely odd one out. You're going to have a hard time with homotopy type theory if you insist on your conventional = definition, for instance.