I don't think that you need to stray from containers at the high school level; they should be more than capable of creating them. I am planning on having my Art I students create containers inspired by a piece of literature (book, poem, short story, lyrics, etc.). I already mentioned the idea to them before break, and they seemed excited about it. In order to make the lids fit, they will put a coil on the bottom of the lid that matches up with the inside of the container. Also, I know that a middle school teacher in the district has done bird houses with 7th/8th grade, and they enjoyed that lesson. If you want to do some sort of cylindrical shape, I've heard that you can just take a hunk of clay, put a dowel through the middle, and roll the dowel until you get your cylinder shape. This method cuts down on your seams, which often come apart.