I’m curious as to how many high schools around the world teach complex numbers. In England in the late 80s when I did A level maths (age 17-18) it was part of the syllabus, but now it is only available in the harder A-level further maths, which is often not taught in the standard non-selective non-private schools, making entry to university STEM courses more difficult.
In Portugal it is part of the standard curriculum although in my time they were poorly motivativated and besides solving some classes of quadratic equations no other applications were mentioned.
The Finnish high/secondary school (= years 10 to 12) gives a choice of two alternative math curricula, the more comprehensive option giving a reasonable foundation for university-level math and is usually very much recommended to anyone interested in STEM. It includes complex numbers (at least superficially) as well as a lot of other stuff that the narrow version doesn’t, like limits, integration, basic linear algebra, and analytic geometry.