Just wondering why you want to teach similar lessons to all the students. From your perspective, it will mean less preparation, but what new things will the students learn? When you have them the following year and you're repeating what they've already had, they're sure to let you know about it.

Maybe you can simplify your program by teaching something like the art elements to all the students. For example, you might concentrate on color, but do a different age-appropraite lesson or activity with each grade. The next week you could do line and then shape, texture, etc.