Best piece of advice my college director would tell his bands was that 'he couldn't rehearse to an empty chair' to them, but also to us as music ed majors, 'rehearse the ones that ARE there.'
I tend to find that I'm pretty pushy on the rehearsals with grading them. If it's a school event it's excused. If it's an emergency one time I'll work with them. Deal with the missing kids separately though. Make the rehearsal with the kids you have positive so that kids don't tell the others they wish they had skipped too.
Hang in there, new school, takes 4-7 years to build the culture. Fight the good fight one day at a time and in a few years this won't be as much of an issue. (It will always be at least somewhat of an issue though)