I am in the same boat. The afterschool program used my art room for years.
Supplies would go missing, kids work laid out to dry on shelves would be
damaged. I finally got to the administrator of the program and we had a
meeting with the afterschool 'teachers'. I told them what was off limits to
students and why. I showed them how the stools were to be arranged every
evening and every morning (they were in there before school, too) We
administer our own afterschool program, and any kid enrolled would be in a
class of mine, so in every class I gave my standard "WE MUST PROTECT THIS
HOUSE" lecture, how they were the caretakers of the room when I wasn't
there - so when they came in for art everything they would need would be
there, we would have enough paper and nobody would be wrecking their
projects etc and to keep a good eye out for kids not being careful. And
absolutely NO BALL THROWING in the art room...I think that was the thing
that damaged most of the stuff.
Every year there were new 'teachers' and they kept getting younger. The
discipline of the kids was always tricky, but I put the sense of pride in
the art room in the kids and it did seem to help. They ratted out a kid who
just wouldn't settle down after school, it was way too loose of a situation
for this kid who needed structure badly. I was able to talk to him and give
him a personal tour of his boundaries. That helped a lot, too.
Now they don't use the room, they put the entire program in another building
with an auditorium. THANK THE LORD!
I feel for you! Make your wishes known and take aside the people who are in
charge and let them know what's what in a way that is not threatening, and
maybe you will get cooperation. I always left little 'love notes' to thank
them when things were going well. I also gave them my used crayons and
markers and some donated paper to add to their own supplies, and let them
use the scissors and staplers and stuff like that. That helped too.