AIUI almost all of the actual mandatory professional education in US law school occurs in the first year, 1L. It's possible and occasionally useful to do more focused professional study in 2L and 3L if you want to but you can equally fill your schedule with Literature and the Law or the like. An MLS (Master of Legal Studies) covers the same material as an Ll.M./1L, is open to non lawyers and qualifies one for nothing.