Have you spoken to your principal about this? Have you spoken to the leader of the group? Why do they need the art room? Investigate to find another room to suggest that they use. Use safety as an issue. You say you are worried about the supplies and the kids in the room, that there may be accidents. Or you say that you are worried about your students ongoing work, you say that the work might not survive the onslought of an afterschool group. You say that you are technically a 'lab class' (like science) and need afterschool time to set up the 'lab' for the next day. You say all of this with conviction, authority, and a sense of indignity (keep the phrase "I dont' think so" as a mantra). Don't be rude, strident or whiney, but be strong, full of suggestions and stand your ground. Try to approach the situation as a 'colleague' and not an 'advisory'. Try to approach the situation thinking "they didn't know" and fill in what they 'didn't know" in a professional manner.
I am frustrated with having to leave my classroom right after school because there is a noisy and disruptive after school program that is now using the room.