According to Wikipedia, Chrome OS usage share is just 0.18%. Yes, that’s just one fifth of one percent. And their K-12 share is nowhere near that high, worldwide.
In the US ChromeOS seems to have about 4-6% market share [0] with about 60% share in K-12. As you say, those are US numbers and the K-12 figures are much lower (single digits) elsewhere. Of course, not sure what availability and promotion to school systems even looks like outside the US.