I might not be the one to address this, as I have never taught elementary school or have had the situation you have. I do have some questions for you and perhaps in asking them, it may prompt you to think about a way to organize.
1. Is it possible for all of the 1st graders (for example) to be working on LINE, and thus narrowing down for you what each "grade" is working on and making it easier for you to remember what they are doing?
2. Is it possible for you to map out your "semester" (or however the time is broken up) NOW, so that you know that the next idea the 1st graders would be working on would be SHAPE (for example), so that if one 1st grade finishes before the next, you know that the next piece they will be working on would be SHAPE?
3. Could you set up each class (again I'lls ay 1st graders) to be associated with an artist, so that for example if you did choose to do LINE with 1st graders, they would be working on something like a Juan Miro or KLee, thus associating each class with an artist in your organization of ideas?
4. Your state should have standards that they expect students to reach certain goals by certain grades (I know New Jersey does). We charted the standards so that we know what is expected for the end of each grade level. The elementary school art teachers concentrate on those goals which are wrapped around the elements and principles, so that all 1st graders would be accomplished in line for example at the end (along with the other art standards required for them to know). So if you see 25 classes a week, and 5 of them are 1st grade, they would all start with line, then shape, then form, then color, etc and art history would be included by associating with an artist for each of the the E & P (which specific ones are expected to be covered by the standards).
My comments are precipitated by the fact that I gave my Beginning drawing students (in high school and they are in grades 9-12) a pretest to see what they know. They were asked to draw a building, a person, a tree, a fence, and a mode of transportation. Their drawings were horrible (and I know that I hear them all say "I can't draw"), without any overlapping, plopped in the center of the paper, no perspective, lollipop trees, box cars, midget people with smiley faces and of course the obligator sun with lines coming off, in the corner of the paper. I will be giving them the same assignment during their final exam at the end of the course.
This pretest indicates a few things to me.
1. Not all students had an art experience recently, and since we have only a 9 week art experience for 7-8th graders, their exposure was limited on that level. Also if you are in 12th grade and you haven't had it since 7-8th grade it's been awhile.
2. Students may be getting the elements and principles, as well as figure work, face work, landscape work, etc, but in a way that is not building one skill or idea on another, but as separate "projects" in their art classes. So when asked (as in the pretest) to gleen what they know (i.e. perspective, figure work, drawing from observation) they can't make the distinction of what the project was actually teaching them.
3. High Schoolers typically "blow off" this type of pretest as a meaningless assignment.
4. I am sure there are a myriad of other reasons why the drawings were awful, but 1-3 are my top three.
I then critiqued their finished work, and in the process said "this is what you will learn to do in this class" as if to indicate, "don't worry about your pretest, you will learn to do this in this class". Their after drawings are always much better, and they are always amazed at what they can do and learn how to do.