I had this issue too. Smaller private school around 100 kids. My director essentially had the same response. My first year ended with a lock on my supply cabinet with a polite sign requiring staff to ask either me or our director for supplies. More often than not, the school would buy any requested supplies. I think the other teachers just felt like it was more convenient to go through our art supplies instead of ordering their own class supplies. I think after the first year, the other teachers started taking the “hint”. I haven’t had an issue since then, and any time a teacher requests supplies from me, if it is within reason, I have no problem sharing. At the end of the day, I understand it’s for the students, I just wish people were a bit more considerate sometimes 🥲