There's no way you can use a calculator like that to figure out your
power needs. You have to take into account the
power handling of the speakers, their dispersion and a number of other factors, like the reverberation or absorption of the gym. I would say that calculator is practically useless in the real world.
If you're running 2000 watts of
power, that means your total PA
RMS should be about 1200 watts. I'll venture out on a limb and assume you're running more than one PA cabinet. So if you're doing one
speaker per side, that's 600 watts
RMS per
speaker. That isn't nearly enough to fill a gym at rock type levels.
Just to fill us in, what equipment specifically are you using for your PA?
As to the job of the mixing engineer. The
producer (in a high school band, then the band themselves) determines the sound of the band. This is usually reflected in their CDs. It's the engineers job to recreate that sound in the space. But the band is usually still the boss. Many times a band will carry mics with them that they absolutely LOVE but ultimitely sound like crap. You can suggest a change, but they have the last word. So really, if it's a situation where the bands the boss, and they're insisting on something that won't sound great, you have to just go with it.
Now high school musicians don't have enough experience to insist on anything. That really makes you responsible for the set up. Insist they don't change their amp levels or that the drummer
play at a certain volume. If they don't, they're really screwing themselves. They ought to rely on your knowledge.