I personally use rhythm exercises to start the class, move on to scales and warm up exercises, then follow that by sight-reading lines or concepts in a method book that are applicable to the music.
It's easy to bear down on parts of a song to work on ideas, but a better idea in my opinion is to give all players the same concept to learn as a group. Once players understand the concept, they will move faster through the music. Typical literature for a 1C middle school band doesn't do much with lows, as they are typically in unison and play very basic rhythms and horn range. Method books can push the boundaries and challenge the kids where typical music that fits the whole of the ensemble may not.