Percussion is one of the easiest instruments to begin, and one of the hardest to master. During rehearsals, be sure that every percussionist has a part. You can double up on mallet parts and accessories. When the concert is getting close, then decide who should cover which parts. Mallets can double oboe and flute parts, with soft mallets. These are much more interesting parts than most mallet parts, and will offer a good challenge.
Have some chairs in the front of band on the side. Any percussionist not playing on a piece should be sitting there. They can read, do homework, or even sleep. It’s important that they are not in the back with nothing to do. Then when the next piece is rehearsed, they can rejoin the section.
One way to challenge your percussion section is to provide a percussion ensemble feature on your concerts. This gives your students a chance to show their skills on accessory percussion instruments. Plus it’s always a crowd pleaser.