The best way to put it about public schools I guess would be, if your kids are the type to stay out of trouble and get good grades, they will excel. It is a typical public school system, where most kids don't try too hard. So the cream will rise to the top. My younger brother and sister went through the Alpine school system. I would expect Marfa's to be about the same.
There isn't a lot of gang activity and violence (probably none actually), but there are a fair share of bad seeds in public school. Which shouldn't be a shock.
As far as private schools? I know there are some montessori schools out there, and some church affiliated schools, but I couldn't really offer an opinion on them.
I don't know if homeschooling is an option, but there are a lot of families that go this route out there because of the remoteness.
If I was going to recommend something to you, I would say go the public school route and see how it works out. If you find a better school that you want to put your kids in then move them to that school. Your kids won't lose the friends they made because the towns just aren't that big.
Anyway, I hope that helps. Let me know if you have other questions. I am obviously envious that you and your family are considering a move out there. Living in West Texas is something that will always stick with me.