My school band of about 30-40 members is in general pretty well balanced, that is of course until you realize there are only 3 clarinets (5 during marching season). The director usually ends up getting the rest of the band to be quieter, but we're still too quite most of the time. Is there anything we can do (tricks, exercises, etc.) to get ourselves to play louder?

Selmer C85 mouthpieces with Fibracell reeds have worked extremely well for me when needing to cut through brass. With this setup you can literally outplay trumpets. Health warning: extreme volume may come at the expense of musicality.

The clarinet has a fairly wide dynamic range (much wider than that of an oboe, for example), but, of course, there are limits. Anyway, the makeshift and arbitrary nature of band instrumentation is one reason I've never written for this medium and probably never will. It's also one reason the existing repertoire is so dreadful. (Another reason for both has to do with the medium's disreputable military history.) My advice is to quit Band and join Orchestra.

The curious thing about your question is that it assumes the onus is on you to balance an uneven ensemble, when the responsibility would seem more logically to belong to your band director. Let's assume he's unable to recruit more clarinet players or to persuade any of the band existing members to switch instruments. Let's also assume he doesn't want to expel any non-clarinetists. In that case, he might just divide the band into two separate ensembles, one more or less a band and the other maybe a brass quintet.

Just out of curiosity, how come those other 2 that only March aren't in the band full time?

<Added>

I meant to de-capitalize March........