Typically, schools that loop do so in fixed-size windows (often 2 years, so K/1, 2/3, etc). Often, teachers for the looped grades plan curriculum together. This tends to minimize the effects of unfamiliar subject matter and the kids' changing social/emotional maturity.

(Source: I helped to start and taught at a charter school in the early 00's. For better and for worse, that school grew into a fairly large charter management organization. Classroom management - which boiled down to convincing a lot of very jaded kids that we actually cared about them - took months at the beginning of each year, and no substantive academic work was possible during that time. Looping in two-year windows struck a good balance between amortizing that cost and preserving teacher expertise.)