Law school isn't like undergrad in the sense that you won't really major in a particular field of law. At most law schools the total number of classes you'll be able to take in a specialty area is maybe three or four. It's typically very limited. Even at a school that boasts a particular concentration, you're still probably only going to get a few classes and maybe a clinic or internship.
I've never heard of a school that specializes in ag law, but Lewis & Clark, Vermont, Davis, Hastings, and Oregon all have better than average environmental offerings. Again though, be realistic about how little difference it makes when it comes to getting hired. I agree with Jack24 that specialty rankings are questionable, and I certainly wouldn't advise going massively into debt to attend a school based on a specialty ranking.