You're in band or orchestra class every day, but we sometimes don't take advantage of the time we're given. Not all of class time is spent playing, so what do you do with the down time?
Listen to your director. When you're not playing, it's usually because your director has to address an issue somewhere in the ensemble. Even though he or she may be talking directly to another section, pay attention so that you don't repeat their mistake, so that you can learn a new method or technique, so you can hear how their part relates to yours, or so that you can get answers to questions that you may have before you even ask.
Work on your part. Not out loud, of course, but take a look at your part. Let your fingers walk slowly over that tricky run on the second page. The next time your director stops, let your fingers go over it again, more quickly this time. Try it at tempo. Work out that complicated rhythm at the bottom of the first page. Count it out in your head, count it in your head while your fingers move over the keys. Find those places in your music that give you trouble and work them out while your director is rehearsing the trumpets or percussion. Just remember to do it quietly so you don't disturb those around you. (In other words, no air through the instrument, no whispering rhythms out loud. It's annoying to your director and to your classmates to hear the ongoing hiss of air through instruments and the occasional chirp because someone was practicing and accidentally made sound.)
Work on other music. If, and only if you are able to play your part correctly, use your down time by looking over other music you have to play. Another piece for the upcoming band concert is giving you trouble, so have it on your stand to look over. Since you always have a scale to work on, let your fingers go over it. If you are preparing a solo for an upcoming recital, work out the rhythms, the tight passages, the phrasing, even memorization. Again, not out loud, please!
A word of caution: When you are working by yourself on any music in rehearsal, keep an ear out for your director. Listen to advice given to another section or to the ensemble as a whole. If he or she starts talking to your section or the whole group, redirect your attention right away. Be ready to play, to resume work in the ensemble; that is why you are there. Be quiet, be attentive, but engage yourself even when your director isn't doing it for you.