If you are a sophomore, juniors and seniors (or really anyone in the band that is older than you) may be less-willing to listen and respect you based on the age difference. Give respect to get respect; do not punish them until they conform; that has the opposite desired effect. Show them that you are mature enough to be standing up there and giving commands, and be friendly. If they are making rude remarks, gestures, or anything related to that, tell your band director. They will always have more power over their students, and they will be able to help in making sure that all your band members will listen and respect your authority.