There are things like plasma and Bose-Einstein condensates, etc. They are harder to explain because they do require a fairly good understanding of the underlying physics which is probably a good bit above the level of most high schools.
Also, easier to explain the basics of phase diagrams with the 3 most common and easily experimented on states of matter. Plasma physics and the physics of low temperature are not easily observed without rather expensive lab equipment (and the people who can run the experiments tend to be expensive to hire too)
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