Unfortunately, tenured or not, most teachers don't have a lot of recourse when it comes to being reassigned. Administrations generally have a lot of latitude when it comes to assigning teaching duties. The only restrictions are if the contract has a "qualified AND certified" clause, which means you must be certified in the given discipline as well as possess certain qualifications, which are spelled out in the contract language. This prevents art teachers from becoming, overnight, music teachers, for instance. The other restriction is the actual workload you are asked to perform, which, by contract, should be restricted as to the number of sections and how large each section is. A good contract will treat all faculty equally in this regard, so you should not end up teaching more hours or sections than the person down the hall. Having said that, not all contracts have specific language that protects those who teach in specialty areas. Your first
instinct about being dumped on is probably the correct one. The longer you are in education, the more you learn that art teachers in particular are often "used" for planning time or convenience, rather than "utilized" for what they bring to the educational experience. Get on your negotiating team! If you don't take an active interest in your own survival, no one else will.