You know, every time I read submissions by members of this list serve with yet another horror story, it makes me glad I retired. What this district is asking of you is absolutely obnoxious at best and probably violates something in your contract. There has to be something in your contract that addresses working conditions and what constitutes a reasonable workload. Most professional educators are expected to perform what is considered reasonable duties, such as making lesson plans, curriculum maps, periodic paper work, etc. The difference here is that those reasonable duties are restricted to your particular job, not the job of someone else! That's like asking the music teacher to do the chemistry teacher's lessons for a year.
What slays me is the almost never ending machinations of school administrators and how they use and abuse art teachers. Is that what they teach in Ed Leadership school? They seem perfectly at ease asking this teacher to take on all the responsibility of two teachers instead of doing what is educationally right--hire her as the art teacher! These same administrators would never hire a kindergarten teacher to teach chemistry or biology, but somehow rationalize that just about anyone can teach art, because, after all, it's just a fun activity that gets kids ready for "real" learning!
When I mention the contract, I do so because a good contract will protect teachers from duties outside the normal routines and scope of their particular job. It should address things like work hours and other work conditions that limit unreasonable duties without additional pay. In my last district, I made sure that specialists were paid for the considerable hours spent putting together art shows, for instance. Check with your union and see what can be negotiated on your behalf. You may be a lowly untenured teacher, but these issues impact everyone down the line--if they can do it to you, they can do it to others, and probably do.
In the meantime, do your homework and find out your rights. Failing that, get out of there as soon as you can afford to do so--this is a lose, lose situation if I ever saw one. You need to be your own best advocate, because precious few will do it for you.