I served as the grievance chair of my teachers union, and I successfully defended a group of fellow specialists who were being told they would have to take on a group of pre-school special needs students (autistic) without additional compensation and cutting into their contractual planning time. The way this case was won was on several fronts: ALL teachers, K-5, were guaranteed the same planning time in the contract, the teachers were contracted to teach K-5, not pK through 5, and some of the students in the autistic program were 2 years old! This was not only a violation of the contract, it was outside the range of expertise of the teacher's certification, something that was also addressed in the contract. Never underestimate the power of good contract language!
If you have a good working knowledge of your contract language, put it to work for you--that is what contracts are for. If language doesn't exist, start pushing the union to get it in there, or serve as a negotiator yourself to make sure it does. If you choose to not be proactive, it is likely you will continue to be less of an art educator and more of a planning time provider. It's your choice.