Why are we not teaching earth science?

The MCAS? I'm not as sure.

First, do read, if you didn't hear, this excellent three part series from WGBH on teaching climate change in Cape Cod schools. The first one (which is what the link goes to) does mention:

Last year was the first year the Trustees ran a climate change specific program, on erosion, for second graders. This is the first year they’re expanding into middle school. They’re able to do that in part because climate change is now officially in the state science standards in Massachusetts, as of 2016

So far, okay. But this line in the third part of the series made me perk up my ears:

One of the main reasons public schools are less likely to teach earth science than chemistry, biology, or physics, is that earth science is not tested on the MCAS, the state standardized test.

Okay, this is not false: there is no earth science MCAS. This sentence gives the impression, however, that chemistry, biology, and physics ALL are required for high school testing, which isn't the case. There are four MCAS science tests--biology, chemistry, physics, and technology/engineering--and students have to pass one. The vast majority take and pass bio, which is why the state has been tossing around the idea of eliminating some of the other tests (you might remember that this came up at the last Board of Ed meeting).
If a student, then, takes three years of science in high school (as most colleges are seeking) or even fills all four, that's two or three years of science that are NOT whatever science is being assessed on MCAS for the student to fill.
The schools may still be choosing not to teach earth science.  It is already the case that not all science is assessed on the high school MCAS, though, so it is tough to make the case that this is why.
(this falls under my general philosophy of making sure you're yelling at the right people)