Teaching multiple classes within a period is not an ideal situation by any means. It is a good way to burn the candle at both ends for art teachers and get two classes for the price of one for the school districts.
I teach in a very small public school, grades PK-12, and this practice seems common. I am the only Visual art teacher in my district. It is against the ideals of good education, and shortchanges the students. However, this practice is done, I believe, because it is the only way to give our student population all the classes that they need within the school year without hiring another teacher or adding another class period at the end of an already full day. There are space restrictions as well. We have very few art electives in grades 7-12, almost none. This means no advanced art program even for the most talented and driven art students. I have been given grades 7 and 8 combined, Studio 1 and advanced studio combined; advanced and beginning ceramics at the same time ... and the list goes on.
I advise anyone who can to discourage this practice. It undermines our discipline and makes our jobs more stressful than they should be, but mostly, it is not an ideal Art education for any of our students who all deserve more.
I would love to hear other art teacher's successful strategies for teaching two separate classes in one period, if anyone has some to share.