I have found reasonable administrators will respond to reasonable rationale that are based on the welfare of the students and the satisfaction of parents.
I would explain that as a professional, I am concerned that our students deserve the best that we can offer them. For the sake of our students, not only for my sanity, I might ask the person who planned this schedule if she or he believes that he or she could do a good job for the students working with a schedule like the one you are given.
I have found that administrators are open to ideas when they are cost-neutral. It would seem that you should have longer time blocks even if you see each student less frequently. I would document the percentage of time spent on getting out supplies, giving instructions & demos, and cleaning up. If you have 40 minutes now, ask for 60 or 80 minutes. See each group less frequently to keep the same amount of contact time (thus keeping instructional costs neutral) with each student. I would point out that all of the extended time is increasing the percentage of learning time because you would cut down on the number of times that students have to spend time getting out supplies, listening to instructions, watching demos, and cleaning up.
To counteract the arguments from classroom teachers who want a weekly break, I would offer a proactive plan to offer more meaningful ways to teach thinking, creativity, including verbal skills (reflective artist statements about their work and written self-rubrics) that are related to their artwork. Learning higher level thinking skills requires more student involvement in the planning and development of their own ideas. Higher level thinking skills are better for life-long learning, are better for overall grades & test scores, and produce minds that can succeed easier in high school and college. Learning in art when approached with good open questions, with creative teamwork, with student designed projects making art choices that build thinking about their other subjects will teach the students how to produce better insights into every kind of problem. It takes class time for students to learn how to observe, to develop their imaginations, to express their concerns, to cri
tique and discuss their work emphatically with each other. Short time blocks have a tendency to drive us to use quick production without much thinking or seeing.
I would not threaten to quit unless I had a better job offer. However, I would certainly leave the impression that I am actively looking for a place that will allow me to do a better job for my students while developing my own emerging ideas on how to be a better teacher.