They have to have rankings because they play in UIL (as a band), other concerts, etc. and some clarinets would need to play first clarinet music, some would need to play 2nd, and some 3rd (if there is a 3rd, some music there isn't). If they all were not ranked in seating, how would they decide who plays first clarinet music and who plays 2nd? 1st is harder, so those that play better are in the 1-4th or so chair and they play the first clarinet music, and it moves on down from there. It would not sound well in a concert if they had people playing first clarinet music that could not play it well and the better players playing 2nd clarinet music. They don't just practice and learn in band- they play concerts, at football games, etc. Just like highschool will.
It's not as complicated as I made it sound probably. They just have several bands in middle school because as of this year they have ALL the 7th and 8th graders for the entire town in the same middle school. Next year however they are going to take away intermediate school (which was all the 5th and 6th graders), put 5th graders back in elementary schools all over town (a choice I agree with!) and they are going to use both intermediate school and middle school buildings to divide up and make it into 2 middle schools, both having 6th, 7th and 8th grade. Doing it this way, I would venture a guess that they wouldn't have quite as many 7th/8th graders in band at each school this way- so they will probably be able to have less "bands" in middle school. Right now they have wind ensemble, symphonic, concert and beginner. I would imagine beginner band would be 6th graders and those starting to play in 7th grade, and with the kids then divided up between two schools instead of all at one- they will probably be able to have just 2 bands besides beginner band, we'll see.
But we had "chairs" even in my day- which was over 20yrs ago. LOL It was based on how you did on the tests for the music they learned. Those that did better moved up in what chair they were amidst their instrument (for instance clarinet) because they tested better on that music and therefore should be the ones playing the more difficult music (ie: 1st clarinet music). It makes for better concerts that way too.