The value of the law is very specific. Conservatives in Texas don't want schools teaching very specific things.
1. Slavery was really terrible and white people did terrible things.
Is that really something anyone doesn't want "taught" in Texas? Genuinely curious, as I was a high school teacher (English, ninth and tenth grades) from 2010 through 2014, and from my experience (admittedly a few years old), this ain't even close to accurate.
Let me post my bona fides regarding this topic real quickly:
I taught at two different high schools in two different cities. One of the schools I taught at was in Dallas, which is a lot more liberal than other places in Texas, but my school, Sunset HS, was 96% Hispanic and thus a lot more conservative (Catholics) than typical. The other school I taught at was in a generally conservative part of Garland, and the neighborhood I personally lived in likewise had a lot more folks who I would guess voted Republican as a rule. (Let me put it this way: every Fourth of July, there was enough gun fire in my neighborhood that you could begin to tell which was a gun and which a firework).
In any case, my experience from those years is that Texans, as a rule, were happy that Juneteenth was a state holiday in Texas long before it has (recently) become a federal holiday and that "conservative" parents and folks were unanimously in favor of their school's history courses covering actual, accurate history, which certainly meant covering slavery and plenty of "terrible things" that white people did. In fact, more often than not discussions about history curriculum would more likely allude to things like that NOT being taught enough. (Edit: as an aside, while I taught English, we certainly taught literature and stories from the days of colonialism and slavery, so these topics certainly came up; plus I obviously had plenty of social studies teacher friends and acquaintances).
Anyway, wanted to add my two cents.
Edit: Thought of one more comment regarding this. A trope I recall several people mentioning when I lived in Texas from those on the right was to repeat that it was the two Democrat parties of 1860 whose platforms either implicitly or explicitly condoned and/or supported slavery, and the brand new Republican party and its candidate Lincoln whose platform was explicitly anti-slavery.* The idea that modern Texas conservatives want to hide stuff like this is rather silly...
*Yes, we all know it's more complicated than this, but as a succinct "gotcha" from modern Texas conservatives, it's isn't too far off the mark, and represents a reason why they were totally keen on real history, including slavery, be actually taught. As I'm personally fond of pointing out, Lincoln's 1860 Republican platform expressly states that a proper interpretation of the Constitution forbids slavery (because of course it did: the institution of slavery was a complete logical contradiction to the American founding documents), and the platform expressly stated that the government cannot give “legal existence to slavery in any Territory of the United States.”
https://www.americanyawp.com/reader/the-sectional-crisis/1860-republican-party-platform/