We are likely producing 'Mattress' at our (new) HS in the fall.  We do not have a band/orchestra.  We also do not have the $1700 it would take to rent the canned music system RnH requires in our situation.

Not having a band or orchestra at a HS is really sad.

Okay... to solutions.  Questions: do you have a MD?  Do you have a rehearsal accompanist?  What kind of music resources did you have on hand when you said "let's do a musical!"?

If you can find keyboard players, synths can cover anything that you can't find in the regular way.  My old high school has used as many as four electric keyboards and sometimes only had bass and drums as "live" instruments (although the band director would usually play trumpet).

When I do my shows, I play whatever is necessary (I'm both choir and drama and have a background in band).  My accompanist plays piano, the HS and MS band directors come to cover woodwinds, and I usually have a student percussionist.  Then we add any students who can hack it through the show, knowing that the adults can help cover trouble spots.  Hopefully this year I'll have a student bass player, or else I'll be stuck on bass.

You might just put the word out to local churches, other schools, etc.  I know lots of schools whose MDs help each other out by playing in each other's pit orchestras.  I may start doing that this year, since a friend of mine just started teaching down the road.  A lot of people will be willing to help the community for free.

If you're near central KS, I'd be willing to come lend my skills (drums, bass, guitar, limited keys) if you cover my gas.  :-)