Hi AMS,
Think BA BOI and Lovethesky, pretty much answered how seniority is decided.
Have to also say, that "being a swat" is not the only criteria for getting a high ranking amongst your group. If you are a geniunely kind, considerate person, able to get along with most others in your group, contribute to group discusions, etc these are always taken into consideration.
Those that attain a high ranking, are usually quite academic, but also continue what they professed at their interview, and not in a phoney manner either. Show other trainees genuine interest, respect other's opinions, coach colleagues who maybe having difficultly, once in uniform follow uniform standards (DO NOT CHEW GUM IN UNIFORM!), be on time for everything, etc...other instructors from other groups can also give your instructors feedback if they've observed a particularly positive or negative action.
Hope this helps!