I fell into this early on, so I feel your pain! There are a few method books/series that have flexible instrumentation (Hal Leonard’s Big Band Play-along and Little Big Band, for instance), but given the breadth of skill level those might not work well for you. Regardless of the style you plan on teaching, you’re probably going to have to do some arranging/rewrites. How comfortable are you with Finale/Sibelius/arranging? I think you have a good instrumentation for some Latin tunes (Oye Como Va, for instance) with the flute, Herbie Hancock and the Headhunters, i.e. Watermelon Man, etc. (for jazz fusion), some easy swing/blues tunes, etc.
For the vocalist I would suggest tunes from Poncho Sanchez’s “Out of Sight” – mostly blues tunes, easy melodies, but you’d have to transcribe them. I’ve been using tunes from that album for my MS’ers and they really dig them!
I really think it’s not as bad as you might think – I’d be optimistic! You can have the flute play melody, harmony on alto and clarinet (any chance they can play tenor?) or have your alto player play the bari to thicken the stack. Sorry I can mention specific arrangements – I eventually gave up on buying charts and now arrange everything due to my “unconventional” instrumentation. Good luck!