Keep Their Attention
It's easier to keep everyone's attention when they are all playing. But in order to fix things that are wrong you need to work with individual sections or players. You can do a little of that in a full-band rehearsal, but if you spend too much time working with one section the other band members get antsy and find unproductive or disruptive things to do.
One way to keep their attention is to have everyone practice silently while you work with a section that needs help. That seemed to work better than just having them sit and wait for their turn to play, but since their attention span is somewhat limited, be careful not to do it too long.
Often the same difficult part happens for different instruments in different sections of the music. Even if they are in different keys you can still rehearse them all together. That allows more students to play and it saves time in the long run.
You can also have all the students practice their hardest part or some specific measure all at the same time. The sound is very chaotic but it can be productive. Fifteen to twenty seconds of it is all I could usually stand and it's probably one reason I lost so much hearing.
The best way to work on individual parts is in sectionals. The older students could occasionally do that during the regular band period. Each section had an assigned rehearsal area (a practice room, the music library, my office, the stage, etc.) and the section leaders ran the rehearsal. During the period I would move from section to section and help where needed. We would usually regroup and play through the pieces at the end of the class period. Additionally, I met with each section before school once or twice a quarter (See Early Morning Rehearsal Calendar).