In many large states (Texas, California, NY) schools are almost fully state funded.
https://crosscut.com/2017/12/oregon-school-funding-education...
https://education.findlaw.com/curriculum-standards-school-fu...
[0] Yes that was an exaggerated response - but not by as much as you might think. https://www.texastribune.org/2019/02/15/texas-school-funding...
Have you looked at a map of where this school is [0]? It's literally between the Washington/Oregon border and downtown Portland. Why wouldn't there be a large commuting population in the Wash State Portland suburbs into downtown Portland?
I'm not deeply familiar with Portland's geography, but it sure looks to me like the urban area goes right up to the Columbia river, so "the traffic thinning out once it leaves the city" would likely be... in Washington.
[0] https://www.google.com/maps/place/Harriet+Tubman+Middle+Scho...
I get that it is coming out of the budget of the school, but ultimately the taxpayers _are_ paying for it, albeit in a round-about kind of way.