This is why teaching at the 3/4A level can really stink. The district becomes just big enough to do other stuff but they aren’t big enough to afford in doing it the right way. If you want to stay, I would:
1) make it a brass only mariachi. (They’re not giving you new money to make it happen, gotta use what you have.)
2) if you have kids that want to sing, choose traditional songs and have them learn by watching YouTube. Make brass arrangements that work for it.
3) if kids want to play violin, invite them to purchase them. Be open with the deficits you have in your abilities to teach strings. Same story with the gambas.
4) have fun with it. It’s not the kid’s fault if it stinks
5) make a reasonable PowerPoint presentation that shows why this will not be of high quality. Include a budget that you believe would be absolutely necessary to make it a good program. Include teachers into this! They need an actual mariachi guy, string program, guitar program and choral program.
6) be upfront with your love for your band program and that being your reason to be there.