I think that some people may be defining "stadium" differently. Probably the majority of high school stadiums in the US consist of a football field with a tartan-surface track surrounding it, and metal bleachers, with perhaps a cinderblock building on one side housing locker rooms, restrooms and a concession stand, with the bleachers and a basic press box above them. Usually the end zones are open, with only a chain-link fence barrier.
Now, that said, there are certainly some high schools that have much nicer digs. The largest in the US that serves a single school is in Allen, TX. It has all the bells and whistles you usually find in a college stadium, and seats upward of 18K.
https://www.dallassports.org/venue/view/20470/Allen-Eagle-Stadium.html?sp=1